Given that Garibaldi Lake is at 1450m, and Whistler is currently reporting 300cm + of settled snow at 1100m, I would think you will definitely need snowshoes.
Skis and a full winter camping set up would probably be advisable.
I went up there on the July 1st long weekend about 4 years ago, and camped on a good metre of snow then.
I would give some thought to snow stability as well, although I don't particularly recall how much exposure one might have.
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