5mm plasterboard? is that right, most plasterboard is either 9.5mm or 12.5mm
5mm - thats got hardly anything to sink into , toggles , gripit will work but thats a very thin board.
If its standard wallboard it will most likely be 12.5 - and as its a toilet roll holder, you know full well that is coming down when someone leans on it too hard when they didn't mean to. Can you not find a stud behind the plasterboard to at least fix one screw to a stud, do you know if the plasterboard is stuck to brick? As drilling into the brick if there is brick would work much better.