Gmail works on your mobile phone! According to Palmdoc, gmail works on your mobile Treo 650.
As for wi-fi, I still can't get it to work on my Tungsten C. I tried both the regular www.gmail.com and m.gmail.com without any success. Maybe they need a special pda.gmail.com for wi-fi users? I'm sure they're working on it already!
Here's a screenshot from the Treo 650 (from Palmdoc):