In America at least we generally use 4’x8’x¾" or ½" to cut corners. Where I live fire resistance is important but I’ve been out of the construction game for awhile so I’m not sure if it’s code here. I do know that in shower areas we use purple/green board for obvious reasons. I live in California now and I suspect fire wall is code here but I’m not certain. I live in a house built over 100 years ago so it’s all plaster.

In general just use sum gyp.