A mineral or mineral variety, or a person who is an enthusiast or follower of Gilbert's work.
From Gilbert + -ite (suffix for minerals or follower of a doctrine). The -ite suffix can mean either a mineral variety or a devoted follower, similar to 'hematite' (mineral) or 'socialite' (follower of society).
The -ite suffix is one of geology's most productive naming tools—geologists have named literally thousands of minerals with it! In this case, it cleverly bridges geology and fandom, showing how one suffix can adapt to completely different meanings.
Complete word intelligence in one call. Free tier — 50 lookups/day.