Dimps developed a "game engine" of sort for their GBA games. As far as I understand SA1 was the first implementation of this engine and then they iterated upon it for their future games. It's an extremely minimal engine, implements some helpers for rendering sprites and backgrounds, and a task system (since the GBA doesn't have threading or any task system in it's SDK).

You can see this SDK in the root of the src, everything in game was written specifically for the Sonic Advance trilogy.

They might have used some middleware, but GBA had no SDK I believe.

The GBA SDK is pretty minimal in terms of library support. It's mostly functions for accessing save data plus wrappers around syscalls found in the bootrom. Those syscalls are some basic math functions, a few decompression algorithms, and an early version of the MusicPlayer2000 sound engine.