Do you mean homebrew or commercial? Sounds like homebrew but eh.
On the commercial I don't know if we ever had any "inside the company" type interviews with those working on the GBA like we see for various really old systems (Commodore SID, Amiga stuff or noted composers like Yuzo Koshiro) say much of anything about music. There are a thousand different sites, even more at the time, so that means little as I would not tend to read such a thing unless I knew the game in question. The GBA was probably the last gasp of such things though for a lot of the old school types so there is also that.
Wandering around various hacking forums and homebrew dev over the years I saw those going for a "that'll do" or transposing certain sounds in their head and just playing accordingly (the A in GBA standard sound font/sample library/sound bank might be different to your keyboard or what was preloaded into your little korg machine in some ways but is still A, ditto snare hit and explosion...). If there was a tool in general then FL Studio (was often still known as FruityLoops as it was during the GBA lifetime it changed and many stuck with older versions for a while) was still pretty popular.
Awave studio had some fans.
You mentioned anvil and while I agree on its nature it is still what was popular among many.
LMMS is something I have toyed with when trialling various pieces of software and a search has it looking like it had a few people crowbar stuff into it, though more for the original gameboy than the GBA.
Openmpt was still largely known as modplug tracker at the time as well. Some did say it lost or altered functionality quite radically.
https://deku.gbadev.org/program/sound3.html
If you are going to ask what versions, plugins and the like will be needed to play (this was all happening so long ago that legacy support of plugins was not the foremost thought in the minds of developers) then I am out.
