Romans fought Gauls and Celts to at least some extent almost continuously in some form from at least the 5th century BC to the mid-second century AD. This covers the early Republic, mid-Republic and late Republic period, and the early Empire, although the biggest and most famous campaigns were during the late second century BC and first half of the first century BC (i.e. late Republic).
Wargaming terms for these periods can vary. While the Early Imperial is generally called that, Late Republic can be "Marian" or "Caesarian". Mid-Republic is usually "Polybian" or just "Republican". Sometimes the period is further divided into "Camillan" and "Polybian", with the former having more primitive armour and more classes of infantry, and the latter being more mail-heavy. The early Republic is uncommon with gamers, as information on the period is patchy and the army wasn't particularly distinctive, believed to have been a phalanx army like contemporary Greeks. (It was a catastrophic defeat by Gauls at the Allia in 390BC. which supposedly prompted the Romans to alter their formations and adopt the famous triplex acies system).
I think the most interesting period of Roman history is what in wargaming terms is the late Republic and this is also one of the most appropriate periods for fighting Gauls in. However this is also probably the most boring period in terms of variety of units in the Roman army. Just cohort after cohort of near-identical legionaries, with only auxiliaries to provide any real colour. In the earlier period you have the multiple ranks all equipped differently, light troops, Roman cavalry, Italian allied legions, even penal legions on some campaigns. In the early Imperial period you have the new units formed by the Emperors like the Praetorians, new cavalry formations, the urban cohorts, and so on.
On the other hand there is something grimly impressive about the implacable shield wall of the Caesarian era just marching over everything in its path. It's what I collect, although I do wish at times I'd opted for 6mm rather than 28mm.
For plastics, there are only really three names in the frame, IMO: Victrix, Warlord and Agema. Agema have a very small range but they offer a few metal add-ons to increase variety. Their stuff is solidly mid-Republican, though there is some leeway on some units.* Victrix have a wider range which is also mostly mid-Republican but with some minor modelling work, some of which could be appropriate as Late Republican (i.e. the Legions of the Republic I box). They also have a new range of Early Imperials. Warlord do Late Republic and Early Imperial.
All three ranges are actually very good with not much to choose between them, apart from Warlord's Early Imperial Legionaries which are very small and dated sculpts now, not recommended. (The Auxiliaries from the same period are fine, though). I have found Victrix and Warlord to be almost completely compatible sizewise, to the point that you can pretty much kitbash between the two. Warlord's poses are more hunched, in an "attack" position, while Victrix's are more static and upright. They complement each other well. I have less experience with Agema but I like their Velites box and although I've not seen the sprues their Legionaries look high-quality too.
Wargames Factory had a range of plastic Caesarian Romans which are now out of production but still not hard to find on ebay or the like. I'm not entirely sure of the details but Warlord is now the sole distributor for WGF and has not made these figures available. I doubt they ever will: they are relatively low-quality compared to the manufacturers above, with much shallower detail. But they are cheap, if you can get your hands on them.
In metals, Foundry have the largest range of 28mm Caesarian Romans. Although I've had some frustrations with them, their quality is generally very high. The odd clunker but that can be spotted from the website. I don't own any of their Early Imperial figures but they look similar in quality and variety. Their mid-Republic stuff is a little more dated. Aventine have an excellent range of mid-Republic metals, some of which can easily serve in the Late Republic with no need for conversion. I'm not sure about Early Imperials but I think Aventine might cover that too.
Warlord have smaller ranges of metals for the Late Republic and Early Imperial periods, although most of the basic troops are not duplicated so you'd have to rely on plastic ranges or go elsewhere for metals. If there are any bad figures in the range I've yet to find them. The only note of caution is that Warlord's figures can be a bit variable in size, which doesn't bother me too much but might you.
Another manufacturer worth mentioning is First Corps. They have a very extensive range of metals across most classical armies including Romans. Their figures are a little smaller than the others mentioned, and they are perhaps not quite up to the standard of the better offerings from Foundry, Aventine, or Warlord. But they have a good variety of poses, scrub up well when painted and are very affordable.
There are countless other metal manufacturers out there, which might be worth raiding for character models or the like. The Mid-Republican period tends to be the most popular in my experience, for some reason. However the above are the ones I'd bestir myself to recommend.
That's all 28mm. For 6mm I'd unhesitatingly recommend Baccus, although not if you need an order urgently as they have some difficulties with supply and demand right now. Hopefully that should clear up in the next couple of weeks. Heroics and Ros also have a range, of course, as do Irregular. H&R figures look decent enough from what I've seen; Irregular are, well, usual disclaimers apply.
I know nothing about 15mm manufacturers.
*Wargamers - and Romans themselves - like(d) to divide up their military history into neat periods, marched by epochal reforms by great men: Servius Tullius, Furius Camillus, Gaius Marius and Caesar Augustus. The extent to which this is actually true is debatable. Particularly with the transition from mid-Republic to late Republican army, it seems there was a much more gradual process of change and you have to squint quite hard to see noticeable equipment differences between troops, the major alterations being in formation. There's also a tendency for manufacturers to base all their figures off a couple of sources: hence mid-Republicans will have feather plumes, Caesarians will have horsehair plumes, and ne'er the twain shall meet. However it is probably best to avoid controversy at first, especially if you're doing it on behalf of another.