You can import any .mid file either into SONG mode or PATTERN mode. I would recommend SONG mode since, obviously, if it is a GM file the author did not break it up into Yamaha PATTERN units (you will have to do that yourself, if you wish).

Press FILE
Press LOAD
Select SONG as the File TYPE
Highlight the .mid file, and Press ENTER to import it to a song location, 1-64. It will take a few seconds to load.
Press SONG and recall that Song location.

If the author of the MidiFile understood and followed the rules of GM you should only have to hit the PLAY button on the sequencer transport and your job is done (sit back and enjoy their music). The GM reset command and the Bank Select and Program Change messages in the first measure will cause the Motif 8 to recall the proper Voices on each part.