Michael Kiske has sung several times with the band. Examples are "Land of the Free" and "Time To Break Free", both from Land Of The Free, and "All You Need To Know" in To The Metal.
Hansi Kürsch also sung in "Land Of The Free" and "Farewell", both, again, from the same album.
Piet Sielck has a singing part in "Watcher In The Sky", from Somewhere Out in Space. Subverted in that the song is taken from Iron Savior's album debut, on which Sielck has most of the vocals and writing credits.
The Pete Best: Although it would be difficult for many fans to recognize every other member aside of Kai Hansen, (and Ralf Scheepers, to some extent, because he went to form Primal Fear) some members are more notorious than others, especially those who played before Land Of The Free was released:
Former drummer Matthias Burchardt is the best example of this trope in the band. He only played in Heading For Tomorrow and the Who Do You Think You Are? EP.
Former drummer Uli Kusch subverts this. Even if he only recorded an album with the band, (Sigh No More) he went later to join Helloween and record the first four studio albums with Andi Deris, (Master Of The Rings, The Time Of The Oath, Better Than Raw and The Dark Ride) before co-founding Masterplan with Roland Grapow. He's also notorious in that he's the only member of Gamma Ray, aside of Kai himself, who played with both Gamma Ray and Helloween.
Another example of the trope is former bassist Uwe Wessel, who played bass in the first two albums (Heading For Tomorrow and Sigh No More) before leaving and fading into the shadows.
What Could Have Been: According to Dirk Schlachter, "To Mother Earth", from Land Of The Free II, was going to appear in Majestic, the previous album.