It's almost certainly a modded or fan-made version (often a modified version of GTA San Andreas or GTA Vice City) that replaces characters, vehicles, or missions with themes from the Bollywood film Gadar: Ek Prem Katha . These mods are created by fans, not Rockstar Games.
I understand you're looking for "GTA Gadar" — but I need to give you an important heads-up.