I installed a Munchkin MK004 security gate in drywall (no studs were available at the location) using the supplied screws and plastic anchors. The gate hinges on one side of the opening and opens at the other. Within a day, the hardware at the bottom where the gate hinges was pulling out of the wall (probably not helped by kids opening it every 2 minutes). I replaced those anchors with molly bolts. The same thing happened. Should I call it a day, remove the gate and patch up the holes or is there another solution - such as using a snaptoggle or screwing a wood board across two studs and screwing the hinge into the board? (Where there are studs on that side of the opening, it is also where there are bull nose corners and I think they are covered with metal and there is no wall on the other side of the opening).
You need to mount it in studs, concrete or something else that can handle the load; drywall by itself will not.
If there are no studs available where you want to mount it, you can open up the wall and either install a stud at that location, or depending on how the gate mounts, you might be able to add bracing between two existing studs and then mount to that.
I wouldn't be too concerned with the metal corner bead, you can screw through it pretty easily.
You might also be able to put up a seperate structure (ie: mounting a metal pole in concrete, etc.) to support the gate.
... screwing a wood board across two studs and screwing the hinge into the board? That's the direction I'd recommend. But not just slats/boards, but rather a single panel, wide enough to cover both studs and the same height at the gate itself. Ideally something like sanded plywood, about 5/8" or thicker, but something like 3/4 mdf would probably be fine, and has a cleaner finished appearance.