The GDM (General DigiMusic) format is a sample-based music format similar to MOD, and is unique to the BWSB sound library. GDM is well known among MZXers due to the high number of songs converted to this format for use in MZX games.
The DOS versions of MegaZeux included a program called 2GDM, which converted several module formats (most notably S3M) into the GDM format. As MegaZeux didn't natively support S3M music at the time, 2GDM was often the only way to allow an S3M song to be used within MegaZeux.
Due to the GDM format's limited usage, previewing songs in GDM format proved to be somewhat problematic, mainly owing to the fact that only MegaZeux and other BWSB programs were capable of playing them. However, wizdom wrote a program called GDMplay to ease previewing of GDM-format music. This program was nominated for Best Utility/Engine during the 2000 MegaZeux Excellence Awards, but did not win.
In version 2.80, when MegaZeux was ported from DOS, it contained utilities for converting GDM music back into S3M at load-time, so that the music could be played back using the ModPlug library used at the time. Since version 2.90b however, MegaZeux uses the libxmp library by default, which is capable of playing GDM music natively. Alternate module libraries supported by MZX have varying support for GDM music.