Personally I wouldn't bother and I'd keep Giga Impact. Why?
Crush Grip has variating damage depending on the opponent's HP - the higher, the weaker Crush Grip is. So as Regigigas does damage, Crush Grip gets weaker - and even when it's attacking a Pokemon at full health, it'll deal more damage through Giga Impact than Crush Grip. Ingame, moves which have recharge rounds aren't as detrimental, since you can switch out immediately if you manage to kill the opponent that turn - so in fact they're decent moves to use. However do remember, Regigigas's ability Slow Start means it's Attack is halved, so you'll be doing half the damage you should be doing essentially - you might want to run a more bulky Regigigas set.