There isn't much content about the period between the Vong arrival and the GCW ending. However, there are three factions of note:
Diversity AllianceConsists of angry anti-Human aliens. Would likely have Clone Wars tech + leftover Imperial tech.
Planets for GC:
- Ryloth
- Storehouse
- Trandosha (A Trandoshan was a high ranking member)
- Chroma Zed
Second ImperiumConsists of rebellious Imperials. Led by Brakiss. Would have the Shadow Academy, which can mass produce Dark Jedi and function as a SSD. Would naturally have imperial remnant units.
Missions and events can be used to reward them with extra units by attacking Kashyyyk, Coruscant, and Hapes.
Planets for GC:
- Dathomir
- Korriban (because darksiders almost always have that)
- Poentially a few planets in the Deep Core, such as Byss or Prakith
Black SunYou know these guys. Led by Czethros, and would have upgraded units from base FOC, with maybe a few stolen Republic ships due to their sleeper agents.
Planets for GC:
- Kessel
- Bespin (The Black Sun was causing trouble there and they need an extra planet)
- Possibly Ord Mantell
New Republic Planets would be:
- Coruscant
- Kashyyyk
- Hapes
- Yavin 4
I'm not sure if GC still has access to this functionality from the campaign, but other NR worlds could open up once the New Republic original planets are conquered. This would emulate the idea of the New Republic noticing that their original task force was wiped out.