i havent get 356mini so i dont know about gasket what actually is ? sry.
It's the red rubber thing on the bottom left corner of that pcb/plate.
The idea is rubber can be compressed/squeezed. So if your plate is 1.5mm, you make the cutout 3mm, then you use a like 1.7mm (how much thicker will determine how stiff the board is, i have no clue how thick is correct, 1.7 is just a random number) diameter oring between the plate and the bottom case. Then when you screw the case together the 3.2mm of plate/gasket get compressed to 3mm, so you get a nice tight fit, and a no gaps at all. But unlike a top or sandwhich mount, that means what your plate is resting on is not metal, but rubber or silicone or something, so you get a softer feel. I think it also sounds different.
That 356 plate is customized more, the gasket is only compressed on the parts that stick out, so the middle of the pcb is free floating, and if i remember right they actually did stuff to the pcb to make it flex MORE for linear switches.
The real advantage of a gasket is it could be tunable, you could have a thicker, softer rubber gasket for a softer feel, or a thiner but stiffer rubber gasket for a stiffer more traditional feel. You could even make cnc metal/plastic gaskets if you didn't want any play.
Here's a simple cross section example: