Hey tombradshaw!
I’ve got GBA working in 3.0B3 (albeit not lr-mGBA as there is a bug upstream somewhere, but a git ticket has been issued I believe).
The only thing I can think of is to make sure that your GBA bios filename is in all lowercase letters. Linux is picky like that (which I appreciate :)
It should look like this:
gba_bios.bin
Of particular importance, check that the extension, .bin, is lowercase and not .BIN.
I hope that helps!
momaw27