What do you mean exactly? The Rebels tech up just fine to level 5.
I even posted a fix in the mod forum to have the Rebel Gallofree Transport actually work at tech 5 also in GC mode.
EDIT:
if you see this in the maps xml
player is Underworld
Starting_Credits> Rebel, 8000
Starting_Tech_Level> Rebel, 2
Max_Tech_Level> Rebel, 4
Then the Rebels are not meant to get to tech level five in teh 'heart of the Maw' GC map.
player is the Rebels
!-- Rebel Starting Forces -->
Starting_Credits> Rebel, 13000
Starting_Tech_Level> Rebel, 3
Max_Tech_Level> Rebel, 4
Yes, it says maxed out. But that is just the default start setting it shows in the Advanced setting menu.
But in game, I can still tech up to level 5 stuff by stealling using the Droids.