Used a molly gun and umbrella fixing before to install tv's and large full motion brackets onto plaster board walls. Used these ones to put up a mirrored bathroom cabinet, they worked great and it's nice and sturdy now.
Firstly, when working with plasterboard, don't use and other type of fixing. The disc ones with 2 fans you open out or poor quality, the tiny flat head screws become worn quickly, they don't fully open out because you can't ensure the inside plaster is flat, and you must drill a, large obtrusive hole to use. Plus their weight load won't be as strong. Don't use the generic cone shaped plaster board fixings for more than a picture frame, they're useless. Some use rawl plugs, they'll also come out.
When using them you drill a hole the right size for the umbrella/plasterboard fixing. Remove the screw, hammer into the hole (should be snug enough you need a hammer) re insert screw but leave enough gap to slot U opening at end of molly gun between wall and screw head. Make sure molly gun is fully closed/reset so both the outer U opening and inner U that'll move with the trigger are flat together. When ready fully pull trigger once, then again, after 2 strong pulls (if the screw was close to the wall) the 3rd should be very tight. Don't over tighten at this point because you can pull the screw out of it's threading. The screw only threads into it's umbrella fixing a little at the very end (but holds fast to good weights).