Trailing 3-1 entering the final frame, the Sens reeled off four consecutive goals to stun New York and claim their third straight win.
Jakob Silfverberg and Zack Smith each scored within the period's first 3 minutes to forge a tie, and Gonchar's tally with 1 minute left provided the knockout blow.
Guillaume Latendresse added an empty-netter to complete the comeback.
Matt Kassian also scored and Ben Bishop earned the win with 26 saves.
Evgeni Nabokov, meanwhile, managed just 18 stops as the Islanders fell for just the third time in eight contests.