Great info. I noticed the disadvantage against archers RIGHT away. Flank dmg confuses me. I'm not sure how this works. My thought was it has a chance to do collateral dmg to the specific unit? However I didn't see this.
The archers disadvantage is pretty bad

. Flank attacks cause damage to a number of same or earlier era siege in a stack if your attacking Horses survive, whether they win or withdraw. Flanking promos boost chances of survival, and therfore make it more likely to flank. Flank attacks can't work when attacking cities or forts.
I think, though am not sure, that the damage they cause is determined by the units damage per strike in combat against the unit it is actually fighting, it was put as a question for a reason

.
What do you mean by, "after the enemy gets spears"? I'll be honest. I don't recall any defensive scores, or how they are calculated. I've only seen STR and figured the % of def. augments X str when defending. Hey, i'm the new guy!! IF anyone wants to explain to me how "def" works, and calculations? I'd be grateful.
Numidians are better against spears as they get a +50% bonus against all melee.
On defense, bonuses increase a defending units base

so spears have 4+100%=8 against horses.
Attacking units bonuses subtract from the defender, in Numidians case, it reduces Spear bonus to 50% (100%-50%) leaving 6v5 in favour of the spear, giving the spear less advantage over them than they have against regular HA (8v6).
Article explaining all combat
Gallic warriors seems comparatively weak to the axe (str 7.5 to the 6 str ga) and not much better than the Sword. Lukewarm at best. G1 makes the unit terrain dependant for an advantage, and while the promo line is cool, It seems like woodsman (if I had to choose between the two) has more utility, and instead your pigeonhold to G1 line. Besides, probably just throw C1 or CR1 on it since it has the same bonus sure the sword. Again, lukewarm.
So to me, this unit is only good when the circumstance is attacking a city from a hill or defending your tile enhancements...on hills. Not saying I can't think of other ways to use it. I just think a swordsman with g1 maybe shouldn't be called a UU.
As TMIT mentioned the ability to be built with either copper or iron is pretty big itself.
Gallics are also the only melee unit that can access the Guerrilla promos, they keep any G promos they have when they upgrade too.
Hills are pretty common on most maps, the G1 alone makes it capable of defending against non AGG axemen at favourable odds on hill tiles, handy but not exactly game changing.
G2 available with a barracks makes Gallics nearly unkillable till macemen (while on hills anyway), but more importantly increases speed by a considerable amount, while not all tiles are hills, the fact they are common means some extra speed is gained (a lot in some cases) hitting faster is quite often better than hitting slightly harder as you will face fewer defenders. You can also pillage the turn you move into a hill.
G3 gives 50% withdrawal when attacking, this means 50% fewer losses when attacking

, the hill attack bonus is just a little icing on the cake.
Hills also cannot be chopped.
On woodsman, forests being more common and AI being typically poor at removing it all you can be sure to gain the defense and move bonuses more often than with G2, however the forests will be chopped eventually, and no strategic resources for pillaging except elephants can be found in forest/jungle, jungle elephants will be chopped early due to the food issue too....
W3 vs G3 is a more difficult comparison.
G3 withdrawal means you will need to spend less

while fighting a war, and will probably mean better experienced troops. The bonus to attacking hills is not eactly a huge bonus though. Also makes attacking without siege a lot more viable.
On the other hand W3s healing allows attacks to continue faster, I wouldn't use W3 on a large percentage of my troops though as W3 doesn't directly help attacking cities, and the first strikes while handy are of most use while defending a forest. They do mean you more likely to damage the top defender in a city though.