Third: Druids are fine. Tree-hugging New Age hippie druids are not.
Yes, save the tree-hugging hippies as the UU for the New Age Pseudo-Environmentalist Civilization...
Fourth: The Celts were known for their excellent craftsmanship (particularly in iron) and their brutal guerilla tactics. I'd propose a UA based on strategic resources.
Specifically, Mineral Resources, since the Gauls were known for their metal-working: perhaps a UA based on extra Production/Gold/Amenity for any Gold, Silver, Copper, or Iron resource in their territory, or simply an Automatic Mine on any such resource, which would give them a fast set of Bonuses in the early game.
Fifth: For UU, I think the best options are a unique spearman, the Gaesatos, or a unique light chariot. The Gauls were well known for their chariots and for their light infantry.
Let's go down the list:
Light Chariot. - possibly with a Menhir-deliveryman riding shotgun, except that the one thing the Asterix cartoons got right was that the Gallic chariots were very well-made and well-designed vehicles for travel
on paved roads, but much too lightly built to survive much cross-country/battlefield travel. The Romans never mention meeting any Celtic chariots in battle until they get to Britain. Not a good choice for a Gallic UU BUT could be reflected in a UA of Faster Travel (All Roads one step higher in effect?) within their own territory.
Swordsmen/Spearmen - two different beasts, the swordsmen are generally considered to have comprised the 'elite' or aristocracy of the stratified Gallic society, while the bulk of the 'rank and file' were spearmen. Generally, the Gauls were always considered noteworthy for the fast movement of their forces and their ability to surprise an opponent - even an opponent who writes up all the history like Jules C. let slip that the Gauls caught him with his Lorica Segmenta down several times.
Suggestion: Another possible UA of All Gallic Ancient and Classical military units (NOT Support) start with the Ranger Promotion?
I suggest what another has already mentioned, let's make the Druid the Gallic UU: a Civ-Specific Guru representing the peculiarities of the Druids, who are described as being diplomats, doctors, architects, and philosophers as well as 'normal' religious priests. Therefore, how about a Guru/Druid who heals military units instead of religious ones and reflects the other Druidic talents as well?
Druid - Comes with X 'Charges' which can be expended to heal X points in every Gallic military unit it is touching at the time, OR expended in a Gallic city to provide X points of Science OR expended next to a City State to provide an Envoy in that City State.
-That should give the average Gallic Player plenty of incentive to scramble for a Religion-Heavy Civ, just to get some Very Useful Druids!
Sixth: For UI, I'd propose the oppidom, an early fort replacement that provides culture and gold.
'Oppidum' is a Roman word, and it does not describe the Gallic construction, many of which, from the archeological evidence, were barely big enough to inclose a single farm, let alone any great numbers of people and buildings. Since many of them are either Trade Nodes/Posts or religious/cultural (votive?) sites, I suggest that the Oppidum provides Fort-like protection, but also 1 point each of Gold, Faith, and Culture in the Tile.
Summing Up:
Gallic Civ UA:
Treasures of Gaul: All Metal Resources discovered in Gallic Territory receive an automatic Mine Improvement
Tour de Gaul: All roads inside Gallic territory are one level higher than normal.
Gallic Civ UU:
Druid: Druids as Guru replacements who can heal military units, provide Science in Gallic cities, or Envoys in City States.
Gallic Civ UI:
Oppidum: a Gallic Fort Improvement that all provides extra Gold, Faith, and Culture in a tile.
Possible Gallic Leaders:
Vercingetorix:
Furor Gallicus!: All Gallic Ancient and Classical military units start with the Ranger Promotion
Divitiacos:
Druidic Knowledge: Every Gallic city produces a 'free' Druid when first converted to the Gallic Civ religion.