Can I mount a workbench on triangular shelf brackets over drywall?
The biggest problem with heavily loaded triangle brackets on drywall is that drywall material is not strong enough to resist being crushed by the lower end of the bracket. You will need to support the bracket with harder material. 

If you have a hole saw, cut some disks from hardwood or similar material the thickness of the drywall. The exact diameter of the disks is not important but should be about 1" to 2.5". 

Make sure the brackets will be aligned with the studs. Mark the positions of the lower fasteners on the wall. 

Use the same hole saw to cut openings in the drywall, over the studs, where the lower fasteners would punch thru. Plug the holes with the hardwood disks. Spackle and paint as desired. 

The hard plugs will resist the force exerted against the wall by the lower ends of the brackets. 

You can also back up the upper fasteners in the same way. This will resist wall damage from anything crashing into the edge of the shelf. It will also make the shelf installation easier.