As I'm SEN, I'm luckily never asked to pay for luggage on LX, so I can't for sure comment on this regarding gate check, however:

I've had it once that at baggage drop, the CI woman immediately told me that my "second luggage" (ie my hand luggage) is too big to take on board (and their wrong-sized metal tray agreed with her) and I'll have to pay for it - she smiled when saying "pay" - I smiled back at her, putting my SEN card on the desk (and remembering her name - my complaint to LX came with my trolley's picture attached with a measuring tape in all 3 dimensions clearly showing I was perfectly fine and telling them to really get their operations all right - got back a "we are sorry that you are sorry" answer, and (I guess just random unluck) then next two times I had to gate check again (before my complaint, it was maybe 1 out of 10 times..)