Veneti as a people^(1) indeed owed a lot of their fame and fortune among Aremorican peoples due to their maritime power; Ceasar describing their ships as follows :
The keels were somewhat flatter than those of our ships, whereby they could more easily encounter the shallows and the ebbing of the tide: the prows were raised very high, and, in like manner the sterns were adapted to the force of the waves and storms [which they were formed to sustain]. The ships were built wholly of oak, and designed to endure any force and violence whatever; the benches which were made of planks a foot in breadth, were fastened by iron spikes of the thickness of a man's thumb; the anchors were secured fast by iron chains instead of cables, and for sails they used skins and thin dressed leather. These [were used] either through their want of canvas and their ignorance of its application, or for this reason, which is more probable, that they thought that such storms of the ocean, and such violent gales of wind could not be resisted by sails, nor ships of such great burden be conveniently enough managed by them
These appears quite close to the IIIrd century ship found at Guernesey in 1982, and that might have looked like this. They would have been particularily adapted to coastal navigation and trade in the Atlantic and Channel, as flat-bottmed ships in early Middle-Ages, in the absence of docks and other harbouring structures. And, indeed, several connections can be made between an Aremorican ensemble led by Veneti and south-western Britain trade points (mainly Hengistbury's Head and Mount Batten). The capacity of these ships to efficiently search an harbour in spite of bad wheater in the various small bays in Aremorica and in Britain might have solidified this commercial importance, Veneti and other peoples becoming important intermediaries between Brittons and high-value Roman and Mediterranean goods (wine in particular) they desired.
On this regard, while Veneti and their client/affiliated peoples were particularily renowed other Gaulish peoples participated into the Atlantic trade, either Celtic (as Pictones or Santones) or Belgians (as Morini and Menapii, themselves regularily allied to Aremoricans) .
Did these vessels went beyond Gaulish and British coasts? It's really hard to say, because there's no archeological or historical sources to back such a statement : it's possible that Aremoricans might have traded with Irish peoples, due to how Romans (such as Caesar in DBG V, 13 situating the island "facing Gaul") but besides a possible presence of "Gaulish specialists" in Ireland (which can be either ill-interpreted or irrelevant to trade) there's no evidence to support what remains a possibility so far apart a later occulist seal found near Tipperary that could be either Gallo-Roman or Britto-Roman.
Arguably, for Romans, Britain and Ireland alike were "uncharted", hence the immense prestige obtained by Caesar campaigning in south-eastern Britain; Caesar's probable reliance on Aremorican (including Veneti that weren't as much destroyed, as their continued existence does points to, than politically beheaded) and Belgian peoples for operations in the Atlantic both against Veneti in all likeness and in Britain would, in a sense, argue in favour of Gauls having knowledge of unknown territories.
Of course, the same could be said of Phoceans after Pytheas' expedition, so I understand your question might rather focus on "uncharted" as "not known by Mediterranean peoples". That, we don't know : again, the lack of historical and archeological evidence is an issue; but the Gaulish ships might not have been really able, nor willing, to give up coastal navigation, bay or island hopping. Even if, technically, nothing would have prevented these ships to go as far as Scotland or Danemark, the distinct lack of evidence such as coinage or pottery found in southern England would rather point to the lack of trade and Gaulish presence there.
Eventually, Gauls don't give us a picture of daring traders and sailors, while skilled marineers : they were rather intermediaries, taking care to control and tax trade flux and taking enough interest to exchange accross the Atlantic themselves.But even during their late independence, they never really went beyond that and exchanges between established partnership (out of proximity, genealogical proximity and mercenariship) and both the lack of archeological evidence of continous trade besides southern Britain, and the make-up of their ships illustrates that. Even the naval battle between Veneti and Romans involved "only" 200 ships gathered from Venetic, Aremorican and auxiliary forces; and remained an unique event during the Gallic Wars.
- Commerce et société en Armorique Romaine; Patrick Gaillou; 1992; Revue des Etudes Anciennes
- Considérations techniques sur la flotte des Vénètes et des Romains in Annales de Bretagne et des pays de l'Ouest ; R.Y Creston; 1956
- Chapitre XXXVIII. La Bretagne armoricaine et la mer sous l’Empire romain In : La toge et les armes : Rome entre Méditerranée et Océan – Scripta Varia I [en ligne]; Patrick Le Roux; Rennes : Presses universitaires de Rennes, 2011
^(1) Tribe in a Gaulish context would rarher be a smaller entity known as touta or toouta, identified either as pagi (probably the case with Ariovist's coalition) or as the "fragments of pagi" mentioned by Caesar, close to Roman tribus or Atheian demes.
