Yes - FPAG is the correct order; GAP was just a term used to discuss the cheaper fares I've seen used and rolls off my tongue a bit easier then PAG of FPAG.
Now, I thought after some time they did start selling G fares with the certificate even if not directly listed. I think the intent of the cert is to be forced into the lowest buckets with the most limited availability. In the case of the reserve cert they hold you to the second lowest bucket in first class and lowest 3 in coach per the terms, which may have slightly more available seating on average but also a higher ticket price which helps negate the value to the user (requires more cash outlay). This then theoretically lowers the actual cost to Delta by earning more income for a companion ticket then allowing the lowest fare bucket to be used. Now I am not saying they are required to, clearly the terms are for an I fare and can be kept that way, but it is just interesting that a perceived higher value certificate doesn't return the same value as say a Plat Cert i.e. being able to get a companion seat in the lowest buckets; not withstanding the many derivatives of YMMV and opportunity cost and so forth. In this case, I would consume 2 Z seats, driving down that inventory along with the correlating LUTXV inventory normally required for Z to be available, and driving more customers to higher cost tickets. Though, if enough people are biting in the I class, the same effect is had plus more money is in the hands of Delta so kudos to them and their data management team.