OLPC project has it! http://wiki.laptop.org/go/Sound_samples (Another gigantic well worth downloads)
Studios and professionals kindly donate it to OLPC and OLPC give it to the public under CC 3.0 BY.
As no one tried to make a GM set in SoundFont or GUS format yet I'm interested to make one as alternative to the improperly licensed freepats project which is under vague GPL. Surprisingly this is seems the only free GM set around since Eric Welsh originating it circa 90s! and went through various license changes.
First I'll use Viena to build SoundFont version then convert it to GUS using unsf? so project like timidity and wildmidi could use it. OPM donated samples was the potential candidate to be a GM though Dr. B said there will a true GM set released soon.