It is unknown who, if any, from the original will return for Gremlins 3. Zack Galligan’s Billy Peltzer may be the one “for sure” back for the third since he was the main star in the first two movies. In fact, Galligan had been one leading the charge to bring Gremlins back to their fans.
His twitter campaign started (or continued) the push to bring Gizmo and company back.