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I think there's an element of (albeit vertical) slump to plasterboard adhesive as the recommended dollup thickness is huge, 50mm from memory. Also not sure why it would require flexibility when cured.
I think I'll just use mortar. Cheers.
SteadyCam-
Flexibility is useful for coping with different thermal coefficients of expansion.
If the filler is lying on a solid floor, in effect a giant heat sink, it will experience less temperature variation than the marble slab exposed to the temperature in your (living?) room.
Now there is a woodburner, (raised legs?, clearance from floor?), confirmed the hearth is likely to experience an even greater range of temperatures.If you do choose 'mortar' do you have any PVA to use in it to help prevent shrinkage cracking?
mespilus
I would think that plasterboard adhesive is designed to be minimal slump. As you dab amounts onto the wall, you want it to stay there, not dribble down the wall.
This means, if you are using it on a horizontal surface, it will not self level. It will also not have free flow under the weight of the marble hearth.
Also it will not have been designed to offer any flexibility when cured.
If you are confident that you can lay the plasterboard adhesive level enough to support the entire surface area of the hearth, and can position the hearth without dusturbing this bed of adhesive, continue with your plan.