The schematic will tell what is inside the GEM pedal, and whether it is easy or very hard to convert.
The fact that there is only a single quarter-inch jack for 6 functions makes me worry that it may be more than simply changed wiring - there may be a microcontroller (a tiny computer) inside. I know Roland footswitch uses a 7 pin connector, and each switch makes contact between the common and one of the pins - that is easy to wire yourself. Not so with a single TRS connector for 6 functions.
If I were you, I'd look on Internet for one of the European GEM retailers, and order this switch from them.
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Regards,
Alex