Gallic armor upgrades. A clone of the standard armor upgrades statistically, but at ridiculously low prices, and only usable on Gallic craft.
This, combined with an across-the-board price reduction on Gallic ships and equipment, will mean you can fully arm a GRN, Council, UC, Maquis, etc. fighter for less than one trade run. Bombers? Also cheaper. Caps? Yeah, maybe lower the cost on the GBs and possibly cruisers - that's up to the devs, I don't care either way.
I figure, make the total price of a fighter, armor included, about 1.2mil. Bombers, maybe 4-6mil. Compare that to Sirian fighters in the 15-20m range, bombers around 30mil. The ridiculously low prices may be a better motive for people to start there, as it'll take almost nothing to set them up.
Fact is, it makes sense for almost everyone to hate Gallia. Twisting existing factions into supporting them would be... unpleasant. I recall there being considerable displeasure from the Junkers about that.
Instead, make it easier for GRN to scramble sheer numbers. After all, if it's so cheap a freighter can pay it, why not slap together a GRN Lynx?
Kazinsal informs me that keeping these armor upgrades off Sirian ships is possible, and if nothing else it could be hooked into technerf.