Just to expand upon zaphod77's comment, the TG300B mode that you see on various YAMAHA devices is basically a GS compatible mode without advertising that fact. It wasn't until late models of MU that YAMAHA officially licensed GS compatibility and could therefore advertise GS compatibility and use the GS logo.
TG300B mode on later devices is literally GS compatible without saying that - the sounds are not the same as a TG300.
The TG300 has the following modes:
GM-A - GM compatible
GM-B - GS compatible (but this is mentioned nowhere in the manual!) - it does contain GS specific sounds and effects
C/M - CM-32L compatible mode
Single - Single channel only, for use with a keyboard.
I like the sound of the TG300 a lot - it's very much its own thing and decidedly different to the later sound sets that YAMAHA used in the MUxx series and later MUxxx/MUxxxx tone generators.
I think that the TG300 is possibly where XG came from - the TG300 has a bunch of effects that would later find their way into the XG standard. Whilst it sounds a bit cheesy and dated now, the Dance demo has a nice wah guitar and some other effects going on. It's a cool little box for sure, but unless you specifically want to get one, it's not a must have. If you want to have a nice selection of MIDI devices and want to add some YAMAHA flavour, the MU100 is a good bet as it offers both MU 80 sounds in the 'MU basic' mode, as well as the slightly upgraded and tweaked MU100 sounds.