Yes, the Gallic Swordsman is useful only during a short time window. If you miss that window, the opportunity is gone. For that reason I like the Iroquois much better. They are agricultural like the Celts, and the Mounted Warrior has quite similar stats. (The extra defense of the GS will never be needed on Warlord...) But the MW can be upgraded to Knight and Cavalry and thus remains useful right up to the beginning of the Industrial Age (when Riflemen become available)! So the MW is much better than the GS, and on top of it: it is even cheaper...
But anyway, you were asking about the GS. Try the following for a successful GS strategy:
- Forget about Iron Working. Instead go for Republic asap. (Read the Military Academy article on the "Slingshot", if necessary.) On Warlord with a decent start position (one food bonus and otherwise average land) you should be a Republic by 1500BC.
- During that time, the capital pumps out settlers, while all new towns build barracks and then veteran warriors.
- Upon reaching Republic, you set research to 0% and rake in hundreds of gold, while still building warriors.
- Even on Warlord some AI should come up with Iron Working around this time. Trade it for Republic and upgrade a few dozen warriors... (Apparently you need early scouting warriors/curraghs to have all contacts by now.)
- If the AI turns out to be a bunch of slow-pokes and no one has Iron Working yet by the time you think you are about ready, then just research it quickly yourself. As a well-developed Republic it should be doable in 4 turns. In any case, you should have several dozen GS, long before anyone has Pikemen...
For an example of that strategy see Megalou's, Neo666's, templar_x's and my game in GOTM91.
(Though in PtW this strategy does not work as well as in C3C: in PtW the Republic Slingshot is not possible (meaning it takes much longer to get out of Despotism and therefore less gold for upgrades) and the Celts are not agricultural (meaning the initial REX is much slower). On top of that, in GOTM91 that strategy was made even more difficult by giving us Carthage and Rome as neighbors...

The GS works extremely well against spearmen, but if you face Carthago's Numidian Mercenaries, Roman Legions or Greek Hoplites in large numbers, the losses will be quite severe...)
Anyway, if you are interested in learning more about this, we could turn this into a
Training Day Game, perhaps in a similar format as Sirian's old classic (see the
rules) so that everyone can get some hands-on experience?! "
TDG -- Making the Most out of the Gallic Swordsman", what do you think?