you'll need to measure the spindle length of the old one that fits.
the BB mounted mech is an E-Type mech. this just has a space created on the outside of the BB shell for it to fit. this doesn't relate to axle length (i don't think) and as far as i am aware, you can fit an e-type BB and use a seat tube mounted mech instead without an issue.
just make sure the spindle length (axle length of the BB) is correct.
sounds like you may need a longer one.
you may be able to use BB spacers, but without pics i'm unsure. also, with it being a GT, there may be something i don't know that a GT nut on here does. i'm sure one will pop along soon to correct me if needed
hope that goes someway to helping!
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