From Mark Supanich, this first-hand answer to the questions I was speculating about moments ago:
I just installed it on my account, and the downloaded Google Gears app informed me that "based on my email volume" it would be storing 3 months worth of emails on my local hard drive. It wasn't clear from the notation if it would be doing that for each browser I access gmail from. There was no option or information (even on the help page) to choose how far back it cached emails on my computer, nor any info on if it would constantly update to clean messages older than three months from the cache. It did note however, that any message in my Inbox, no matter how old would be cached, and that spam and items in the trash would not be cached.
OK, total of two minutes taken away from "real" work! Turning off email account for next 12 hours or so. Thanks for the clarification.