Not sure what that bracket is. Probably would not try yourself but ask your installer. If it extends behind the sheetrock, it should have been removed or at least covered in some way.

Good luck with this one. What brand are the windows and how was the install (full frame replacement/new construction?)

Were these windows strapped in place and this one utilized properly? Thinking may be a masonry strap bent in instead of strapped to the structure. It may need to have the window removed for that "brackets" removal.

There are all kinds of creative solutions out there.

Why wouldnt the contractor be responsible for fixing this?

I suspect the bracket was intended to be bent flat against the framed opening and secured with a screw. You should be able to cut away the drywall, bend the bracket, add a screw and then patch the drywall.

Removal is likely to damage the window.

Can we get a zoomed in pic?

Do you plan to have window trim or just the drywall return?