If it was working, but now it stopped, first step would be restarting the computer.

If that doesn’t help, try exporting the doc as XML, and then import it back into Finale.

If still no joy, we would need to look at the doc. You can’t post a Finale document on this site(!) so put in in Dropbox and post the link.

Have both staves been assigned to the Marimba? Don’t assume this happened automatically.

Also, check the range. GIFF and also GPO are fussy and limited in terms of instrument ranges (I've literally had to edit the samples to permit certain string notes that are very much possible via harmonics but were considered "out of range" by Finale and did not scale within the samples without some manipulation).