I've been working with GTA for a few days, and this is what I've found out:
The eboot.bin is encrypted (Thanks to Erant for figuring out this)
Most likely, it's the gameboot prx from 2.0 that is used to decrypt it.
So, to get it working we would need to:
-Get the key from the 2.0 PRX, Use it to decrypt the eboot.bin, and repack the ISO. I dont like the idea about this, because, it means
ONLY ISO's would work, and the UMD's wouldn't. Also, it means, we would need to patch all new games coming out with this protection.
-Get a fully working emulator, to emulate the 2.0 PRX. Replacing the gameboot in 1.5 with the gameboot from 2.0 would not be a good idea.
Because, the gameboot PRX is also the file that is used to lauch homebrew. So, if we replace it with the 2.0 PRX, most likely GTA would
run fine from a UMD, but homebrew wouldnt work. Also, you wouldn't be able to run the ISO, because you wouldn't be able to lauch an UMD emulator.
For me, this seems like the 2 only ways to get GTA working on a 1.5. There is also one more way to get the ISO to run thou...
-Run a UMD emulator on 2.0. If we were able to do this, the ISO would run fine. To get this working, we would need kernel mode on 2.0,
which at the moment is impossible.
I hope this will help somebody out there who's working on it, and I will still continue on my work.