The plasterboard itself won't hold that sort of weight reliably, so you will need to fix it to the studwork behind.
If you can't attach it directly to the studwork, there are ways around it:-
1. Screw a sheet of plywood or a couple of, say, 4 X 1 timbers through the plasterboard to the studwork, and fix the heater to that.
2. Cut out a section of plasterboard behind where the heater is going, fix a couple of noggins between the studs to take the fixings, replace the p/board and make good.
3. Replace a section of p/board with plywood
4. If it's a brick or block wall with plasterboard dot & dabbed over it, drill through the board into the brick / blockwork. for the fixings, and use standoff spacers through the p/board