I am a big fan of the ass-gape. It's totally cool in str8 prOn. However, I think trannies have a disadvantage here bc they simply don't have the mid-body of genetic females. Theirs is "designed" to give birth, so I suspect (less) muscles, tissue etc is more flexible, thus resulting in GG's gape well when fucked up the buns correctly.
That said, there is quite some gaping tranny ass out there on the good ole trannysurprise site. Especially the older movies.
A few pix from there