I would highly suggest watching this video if you plan to buy one. It's an exhaustive review of the GMMK Pro with a bunch of tips on modding to try and add some flex to the plate and PCB. In my opinion though, modding it for flex is not worth the effort. It's not enough to make a meaningful difference and stiff kits aren't inherently bad anyway. That's purely preference, and the GMMK Pro is a perfectly fine kit so long as you replace the shitty GOAT stabs with something better like Durock V2s. If it comes with their newer GSV2 stabs then those might be fine too.

More importantly though...just get a Keychron Q1. The Q1 doesn't do anything fancy or new either, but I've used both kits and held them in my hands and the Q1 just looked and felt better in every respect. So if you like the black, silver or blue colors Keychron offers, then get the Q1. The GMMK Pro used to be competitive, but that's not really the case any longer. The Q1 is in the same price bracket and is pretty much an identical layout if you get the knob version.

If you specifically want a bouncy gasket mount kit, then consider the Tiger Lite from KBDfans. It might be a plastic kit, but it's a proper gasket mount kit with tons of bounce, and it has some cool case color options to choose from. I think they just restocked all the colors too.