Thanks for providing more details 
Some things upfront:
- Now it is clear that your problem only concerns the use of the Keylab with a specific pedal and is unrelated to Gig Performer itself. As in my experience, this is a very friendly and helpful community, you‘ll get some help here, but I‘m not sure if we have any Arturia hardware experts here. You‘re more likely to talk to those on channels dedicated to Arturia stuff. I‘m not saying you must or should not post it here but also do not be disappointed if you don‘t find the right people here
- To make your topics on this forum easier to find, it is helpful to post them into the appropriate category, which would be Hardware. Seems banal but actually helps people orient
- I assume you‘re talking about the Keylab 88 mk2, right?
Now, regarding your two questions:
What is the behavior when you plug the pedal in the Expression port?
Of course, there are workarounds to this, either in hardware (using a polarity adapter, resolder the wires in the FC3A, use another pedal type) or in software (using Gig Performer to transform the reverse sustain input, similar to this: Next/Previous Rackspace double jumps - #4 by ezraherman)
But it’s frustrating that this is actually a problem with a MIDI controller released in 2019 
Did you try to save your settings as a user preset?