I'm not aware of any reference in any ancient source to this sort of thing, although I won't rule it out completely. But let's have a look at where the Wikipedia article on the film Gladiator, which is the linked source to the TIL post, gets its information. Ah yes, that reliable historical source, IGN:

There's another historical reference that didn't make the transition from the written word to the big screen, and I can see why. In the script, Maximus did a product endorsement for Olive Oil in the ancient Roman City and had his name plastered on billboards across Italy, but this might not be as far off as you may think. Today, in our sophisticated society that has seemingly evolved so far, we find ourselves surrounded by famous people pushing products, and this is actually how it all began. Although now we find ourselves listening to Michael Jordan telling us what shoes to buy and Grant Hill suggesting what we drink, it's the same principle, and it all started way back when.

This doesn't actually say anything at all. No ancient source is mentioned, it just insists "this is actually how it all began." The idea that a gladiator would promote olive oil seems very dubious to me, because olive oil was such a staple in the classical Mediterranean. It was used for food, medicine, bathing, and lighting, and it has therefore been estimated that each inhabitant of Rome would get through at least 20 litres of the stuff per year, probably more. In the Severan period, shortly after when the film Gladiator is set, oil was added to the state dole, so that any citizen could now claim it for free. Perhaps the article means that gladiators would promote a specific "brand" of olive oil, but brands didn't exist in the same way as they do today. Amphorae might be stamped with the location from where the oil came, but there's no indication that the producers went out of their way to promote their own produce as better than the rest.

Of course, it's dangerous to argue from silence. Just because there is (apparently) no evidence that gladiators promoted products, this does not prove it didn't happen. But I remain sceptical, largely because commercial advertisement in general doesn't seem to have been much of "a thing" in ancient Rome.