Actually, I just tested it and there's a third factor I'd forgotten about - EVERY civ (Imperialistic or not) gets an intrinsic +100% Great General point bonus when fighting inside their territory. This bonus isn't really documented very well, so it's easy to forget about.
The full (corrected) table would be:
No Imp, No Great Wall, battle outside home territory => 1 XP gives 1 XP on GG bar
No Imp, No Great Wall, battle inside home territory => 1 XP gives 2 XP on GG bar (+100%)
No Imp, Great Wall, battle outside home territory => 1 XP gives 1 XP on GG bar
No Imp, Great Wall, battle inside home territory => 1 XP gives 3 XP on GG bar (+200%)
Imp, No Great Wall, battle outside home territory => 1 XP gives 2 XP on GG bar (+100%)
Imp, No Great Wall, battle inside home territory => 1 XP gives 3 XP on GG bar (+200%)
Imp, Great Wall, battle outside home territory => 1 XP gives 2 XP on GG bar (+100%)
Imp, Great Wall, battle inside home territory => 1 XP gives 4 XP on GG bar (+300%)
This is confirmed with in-game testing.