G. Mandel was a 22nd century member of Starfleet, one of the personnel listed on the dedication plaque of Enterprise NX-01. Mandel was listed in the group of names of representatives responsible for the vessel's flight software. (Star Trek: Enterprise set artwork)
This character was only mentioned in writing.
Mandel was named for production staff associate Geoffrey Mandel. This name was seen on the Enterprise (NX-01) dedication plaque. There are other, later, members of the Federation's Starfleet named Geoff Mandel.