Why do you hit the gates in the giant slalom?

I think you may be mixing up two different Alpine Skiing events.

Giant Slalom is a course where gates are more spread out. Two vertical poles with a net in between generally make up one gate.

Slalom courses are closer together, with one vertical pole making up each turn/gate.

In Giant Slalom, a racer may hit the gate with their hip or pole, usually when in a tuck.

In Slalom, skiiers cross-block gates with their poles or shins as they are finding a shorter line and setting up their next turn.

Not the best answer, but close enough.