You should get a mapping achievement for visiting GM gaol.

[Enkidoh]

Irony is a past Rising event many years ago had players actually go to an instance of the GM Gaol (Mordion) legitimately as part of the event (and met the Wandering Minstrel in there who was also imprisoned there - it's a long story) - and it had all the restrictions of the actual Mordion (communication other than say was disabled, you couldn't use teleport/return while there, abilities and items were also disabled.).

Alas there was no achievement specifically for visiting it, but getting the achievement for completing that event quest sort of qualifies by default.