.GBM File Extension

I have two very important files with a .gbm extension. It's a back up someone did on what used to be my computer, I deseperatly need it. The thing is I don't hace any idea how to and I couldn't finde anything on line. Can anyone help me, please?
TXS

its used in "film-play" on a gameboy, with a SD card reader (film play is used to see movies, listen to music, and see Ebooks on the gameboy). I have an converter to convert music, videos, and pictures, to gameboy formats.

G-B-M.= Game-boy-music.
G-B-A.= Game-boy-audio.