Ah, I see now. It all clicked when you said these two things:
"the iRule grabs the IP's in the response and translates those"
"where the key is the internal IP and the value is the 'external' ip"
However, in the iRules section of the GTM there is no Data Group List tab like there is on the LTM. I don't understand what you mean when you say:
"Note that although developed for GTM, this is an LTM iRule and is attached to the LTM VS that is created by the GTM when you configure a listener address."
Could you please elaborate on this? On a GTM, the only place to assign an iRule is on a WIP (at least that I am aware of).