What time period are you talking about? The Gauls were a collection of Celtic tribes inhabiting central and western Europe (Spain, France, southern Germany, Northern Italy, and England to put it in rough modern terms) pre-Roman conquest. The Goths were a germanic people who migrated into the Roman Empire from east of the Rhine in the 4th and 5th centuries CE and played a significant role in the destruction of the Western Empire.

You are not wrong that Roman superiority was less than pop-culture might have you believe. There were a lot of reasons for this. But you may also be operating under a some misconceptions.

1] The Romans were not that technologically superior

First of all, we are talking about late iron-age armies here.

We live in a day and age when technological progress is an every day occurence. Be it smart phones or smart bombs, we are used to technology being "better" than it was just a few years ago, and that having better technology gives great advantages.

This was not the case 2000 years ago. People still invented things, people still improved and refined existing techniques, but by and large the technology of a soldier serving under Justinian in 560 would use much the same kinds of weapons and armour as a Roman citizen serving as a militia soldier at the height of the Republic in 250 BC. And a soldier serving under William the Conqueror in 1066 could still be using very recognisable equipment: They'd all be using some variety of spear, large shield, and wear mail armour if they were lucky enough to be able to afford it.

Spears, swords and shields were already ancient when the Romans took them up. It is not entirely clear where mail armour came from, but the common theory is that the Romans copied it from the Gauls. Yes, the Romans LEARNED their technology from the so-called "barbarians." Likewise, their famous short-sword was called the Gladius Hispaniensis, or "Spanish sword" and may have been copied from Iberian mercenaries serving Hannibal. Another "barbarian" invention. The Romans were quite proud of their adaptability and skill at using other people's wisdom. This should already reveal a good deal about the balance of power.

2] Technological superiority does not matter all that much

We're talking about "who has the pointiest stick" here. An army that uses a slightly different type of sword or slightly better type of body armour isn't going to fight any differently than an army that doesn't. When you're using hand-held weapons and shields and facing eachother on a muddy field a few miles across, other factors matter much more: terrain, numbers, morale, leadership, how well rested your troops are, how well fed they are, how well motivated they are, whether the omens are good, what individual warriors decide to do on the field, plain luck.

This means that battles in antiquity, even more so than now, are contingent. No army is going to win all its battles. Those that famously do win a streak of victories, such as Alexander the Great's or Caesar's armies, generally share many of the above mentioned factors throughout their battles. (Same leaders, motivation and confidence that grows with each victory, enemies that become more fearful as their reputation grows, etc.)

Vastly assymetrical warrior-cultures could have difficulty fighting one another, to be sure. Crassus found this out to his peril at Carrhae, where it turned out his legions were ill-equipped to fight mounted archers on an open plain. Earlier the Romans had struggled mightily against the guerilla-tactics used by the Iberian tribes. But these are more questions of culture and terrain than of technology. (The Romans had decent bows and fine horses. They did not have large numbers of men capable of riding them and shooting while moving.)

3] The Gauls were not undisciplined, ill-equipped, borderline hunter-gatherers

Like I indicated above, they were the ones who may have invented mail armour in the first place. Gauls are not my speciality, but they were great builders of fortresses, had a rich culture, had long traditions of political leadership and organisation. The comic-book image of the "barbarian" in shaggy hides grunting and smashing things with clubs has no basis in reality. (Though it has some basis in Roman prejudices.) And even then it should not be applied to the Gauls in particular.

Roman and Greek writers who describe the Gauls in battle do mention a number of stereotypes. Gallic long-swords, according to them, were often of inferior quality and bend during battle. Gallic warriors were fearsome on the charge, but grew discouraged quickly and ran when fortunes turned against them. It's hard to say whether these stereotypes had much basis in reality. (Not impossible, mind you. The Roman organisational system did probably afford them much greater staying power.)

But let's look a bit at how our primary sources describe battles against these "barbarians." I'm taking episodes from the later Republic here, mostly because that's where these stereotypes originate and also because that's where we have our best narrative sources.

Let's start with Polybius. He's a Greek historian who had personal experience with the Roman armies, and sought to explain to his (Greek) audience how the Romans had come to dominate the world. Polybius is a writer who likes weapons and equipment, and often explains victories by stressing differences in equipment. (I personally feel he probably over-states this impact.)

Very terrifying too were the appearance and the gestures of the naked warriors in front, all in the prime of life, and finely built men, and all in the leading companies richly adorned with gold torques and armlets. The sight of them indeed dismayed the Romans, but at the same time the prospect of winning such spoils made them twice as keen for the fight. But when the javelineers advanced, as is their usage, from the ranks of the Roman legions and began to hurl their javelins in well-aimed volleys, the Celts in the rear ranks indeed were well protected by their trousers and cloaks, but it fell out far otherwise than they had expected with the naked men in front, and they found themselves in a very difficult and helpless predicament. For the Gaulish shield does not cover the whole body; so that their nakedness was a disadvantage, and the bigger they were the better chance had the missiles of going home. At length, unable to drive off the javelineers owing to the distance and the hail of javelins, and reduced to the utmost distress and perplexity, some of them, in their impotent rage, rushed wildly on the enemy and sacrificed their lives, while others, retreating step by step on the ranks of their comrades, threw them into disorder by their display of faint-heartedness. Thus was the spirit of the Gaesatae broken down by the javelineers; but the main body of the Insubres, Boii, and Taurisci, once the javelineers had withdrawn into the ranks and the Roman maniples attacked them, met the enemy and kept up a stubborn hand-to‑hand combat. For, though being almost cut to pieces, they held their ground, equal to their foes in courage, and inferior only, as a force and individually, in their arms. The Roman shields, it should be added, were far more serviceable for defence and their swords for attack, the Gaulish sword being only good for a cut and not for a thrust. But finally, attacked from higher ground and on their flank by the Roman cavalry, which rode down the hill and charged them vigorously, the Celtic infantry were cut to pieces where they stood, their cavalry taking to flight. - Polybius, Histories, Book 2, XVII

Here, Polybius claims that the Gallic shields and swords were less practical than the Romans', and that those naked warriors up front were vulnerable to javelins because they didn't wear any armour or clothes. There may be some truth of that. I certainly question the wisdom of fighting naked.

However, the actual battle has the Gauls fighting the Romans on equal terms until the Roman cavalry wins the day. (the stereotype of the Romans being bad with cavalry doesn't have much basis in fact either. It was only later that they started relying on their allies and auxiliaries almost exclusively for cavalry.)