I would hope you're right, except: 1. If all they're after is an assertion that you've used Gmail in that time frame... well, that's already implied by the fa

I would hope you're right, except: 1. If all they're after is an assertion that you've used Gmail in that time frame... well, that's already implied by the fact that you've received a notification of the class action and are replying to it. 2. The notice is written colloquially, not in lawyer speak, and the audience is the general public, so it seems more reasonable to take "proof" to mean "evidence" here.