I know you said GRN but I'd still advocate for Core because it seems like they might bring the Omicrons back alive with activity (possibly).
Gallia's been around for almost 7 or 8 years and it's still inactve. Core has had more activity in 1/4 the number of years that Gallia has existed. So I'd say Core, especially since their activity in the Omicrons means more Zoners, Corsairs & Order will log too.
Core, Zoners, Auxesia, Order, if you're interested in less intensive stuff and PvE in the Omnicrons.
LibNav, Bretonian Armed Forces, Gallic Navy, Council, Crayterian Republic, if you want more intensive stuff and PvP in the Taus/Bretonia/Southwestern Liberty/Gallia.