I'd be inclined to find the studs and screw into those, if necessary fixing some battens or even a piece of ply to the wall to mount the bracket.
Plasterboard plugs are OK if the load is mostly vertical but as soon as you hang it out there's a good pull on the top fixings. Plasterboard will bow even if it doesn't break. Try leaning a piece against a wall for a couple of weeks, it will bend with its own weight.
But I don't do this sort of thing for a living.
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