Ever since i first saw the movie in 1983, i have hated that scene. A gonk droid feeling pain? Seriously? Pretty much sums up the things wrong with that movie.
I really hate to say this, but droids feeling pain has already been established as canon in ESB Revisted. At 1:56:44 when R2-D2 is welding C-3PO’s foot, Threepio says, ‘Ouch! Pay attention to what you’re doing!’ when Artoo’s welding goes slightly awry.
If Threepio can feel pain, it’s not unreasonable to think other droids can as well… even gonks.