The whole Prozac thing is problematic. Many people get discontinuation / withdrawal effects for a long time after they stop. So what happens is they stop long enough for the meth or other drug to give then a high, but the anxiety/panic that are part of the discontinuation effect is still there. So they get a fucked-up anxious high.
Also, an ounce (28 grams) in 5 days is a truly fucking epic quantity of even crappy meth for a casual user. Even at the end of a one year almost daily IV habit I was only doing 1 to 1.5 grams in 24 hours. And having at least a 2 day break every week. So 5 days would usually never be more than 5 grams.
It is possible that your Prozac was increasing your perceived tolerance and allowing you to consume way more than normal without getting too high. This used to happen to me with antipsychotics. However, when such drugs block the euphoria they don’t block the cardio-vascular effects meaning you still can overamp and get anxiety and panic amongst other symptoms.
I can’t comment on Gabapentin because I basically have no idea what it is or what it does and it seems to be a thing opiate guys are into.
Basically all the info in this thread suggests to me that Meth is not a good drug for you while you have a panic disorder and are on an SSRI. However, if you can isolate what it is about the Meth experience that you really like and are after maybe there is some other drug that can get you where you want to go? For example, I never really cared about the euphoria so much as I liked the pro-sexual effects and the sense of calm concentration and focus that meth gave me - so it was easy enough to substitute dexamfetamine, viagra and valium in low doses when I wanted that kind of party.