Since GBA can't handle PC/PS2 maps
They were obliged to make these maps as close as they can to the other ports
You can notice that there are some similar places like the end of Gaul in PC/PS2 and GBA (both versions you need to go through the wooden building, go straight the alley and pass the small bridge to save Getafix)
It's like the GBA maps are overviews of their PC/PS2 counterparts
But the strange thing is that Gaul in GBA have a stage with Obelix acting as boat and Asterix riding him through a river
This stage doesn't have a similar part in the PC/PS2 versions
Maybe this part was planned for PC/PS2 before the GBA but they hadn't the time so they dropped it.. and made it on the GBA version instead
Considering the fact that all GBA sprites were taken like stop-motion of their 3D PC/PS2 animations counterparts
The 'Obelix acting as a boat and Asterix riding him' animations maybe were already before in the PC/PS2 versions too but were cut/replaced by Obelix pushing a boat
Hope somebody understand my shit 😂
The GBA version was made during/after the other versions, by 2 completely different developpers, Ferando Velez and Guillaume Dubail.
What they got was assets from the console version to work with basically.
Sounds, character models, textures, etc
I don't think there was ever a plan to make a river part in Gaul, as it was always meant as a short tutorial level.
VD-Dev simply expanded some ideas.
Altough, I wonder if they got their hands on beta Rome, because the GBA Rome shows a similar overview structure with the beta.
What I think might be possible about the empty Gaul file would be a forest section.
The vertical slice video from 2001 showed Asterix & Obelix in their forest before chasing a roman to the outside
So there's high chances that early in development, we were meant to control Asterix & Obelix doing their hunt in the forest while the bad events occur, and that may have been scrapped for limitations of their engine?
I guess what you said might be right about the forest part