Good point Jazzmammal-
From the Manual-
"You cannot enter GM mode using solely the RD-700 on
its own. The RD-700 is switched to GM mode upon
reception of a GM System On, GM2 System On, or GS
Reset message, which has been supplied to it by being
contained within the setup data placed at the beginning
of a song that is to be played back.
• You cannot switch to GM mode if the Edit screen’s
System settings, or the Rx GM System ON, Rx GM2
System ON, or GS Reset settings are set to “OFF.” For
details, refer to
“Switching Between Reception of GM/
GM2 System On and GS Reset”
(p. 70)."

These are in the main setup edit section of the synth.

There is quite a bit in that manual about using GM...

I do not work here, but the benefits are still awesome
Make your sound your own!