Gasket basically means that the plate is fixed by pressure instead of screws. Like the custom keyboard MEME.
Here is a picture of the meme in a explosion view. Since i don't really have a gasket mount keyboard this is the best i can explain there is probably someone else here that can explain it in more details.
Why its more expensive its because only super high end keyboard use this new mounting like system what also i presume makes the CNC more complex what makes it more expensive to manufacture.