GM = General MIDI. Meaning any standard MIDI file you find will play on this.
Now, 90% of GM modules are simply a collection of sound samples on a ROM chip, meaning each module
will sound different. (MIDI files are essentially digital sheet music.)
As for the MidiWorld, NEC was not a music company, but they partnered with Roland quite a bit on PC-98 series. Considering this device is pure GM and does not support GS, I would guess it is actually a Roland SC-7 on the inside.
If you are curious about MIDI, here is a simple example. Both videos are playing the X68000 version of Super Street Fighter 2. You are hearing the exact same MIDI files, but you will notice they sound very different from each other, as they are played on modules from two different companies.
Yamaha TG100 (1993)
Akai SG01k (1996)