GBA+opengl, is it possible?
Probably not. The GBA is a nintendo proprietry device, and will most likely use a nintendo proprietry API like all their other consoles have done.
Google is your friend: http://www.ece.uic.edu/~msripada/tgl_home.htm
A full OpenGL on GBA would be extremely slow. Even a subset would be sub-optimal for the GBA’s design.
The best way to do 3D graphics on GBA is to design and implement everything from the scanline rasterizer up with exactly your game in mind.
Imagine this, the fastest you can clear the screen to a single color using the best assembly language routine is about 10 million pixels per second. This is the upper bound of your fill rate, and in a real engine you will be lucky to get half of it.
There is no official API for the GBA, you program it by directly poking values into registers using C and assembly.