Some time ago Google started to detect my p1i on the download page for the mobile gmail app. Unfortunately the p1i version does not work correctly on a p1i phone. No fullscreen and the back button does not work.
Fortunately it looks like the version for SE k750i works correctly on SE p1i. I got it using the following URL:
Update 20080409: The above URL is now broken, and Google has fixed the fullscreen issue with the P1i version of Gmail j2me but not the back button.