I don’t own a Geissele rifle but from what I can see from these specs, their reputation and current market availability, I don’t think you’d go wrong ordering either of these (depending on your budget).
14.5 Pinned
https://www.primaryarms.com/geissele...ck-14.5-pinned
16
https://www.primaryarms.com/geissele...-16.25-08-188b