Think I saw a post from WoodYouLike recently that advised if you must use wood in a bathroom (not a very good idea IMO), use only engineered wood, solid wood may be to dimensionally unstable in humid conditions. Have a search through the Floors, Stairs & Lofts forum, I suspect it’s in there somewhere. I agree with the others on the rest of it but, additionally, the weight of the plasterboard on the edge of the floor may be enough to stop it expanding & then you’ll have huge lump in the middle!
If your using plasterboard in a bathroom, you should only be using Moisture Resistant; if you want to skim it, it must be primed first; if using plasterboard in a wet area, it should be tanked or there is a good chance it will dissolve behind the tiles (assuming your having tiles); better still use a waterproof tile backer board in wet areas.
Take some time to read the Tiling Forum Archive & Tiling Sticky, it may save you a lot of heartache.