If you go to http://gmail.com/app on your Treo web browser it says there isn't a native version of Google Mail for your phone but that if you're certain you have Java installed you can download it anyways.

So I went through the process of installing Java and then the Google Mail app and because the process differs greatly from the instructions you'll find online I've recapped the steps I took here:

1. Download the JVM for your Palm from here. The Mac version is a SIT file which my MacBook Pro doesn't recognize - no matter, since we're only interested in a couple PRC files inside, just get the windows ZIP file.
2. Uncompress the ZIP and install the two PRC files on your Treo. Now I don't have Palm Desktop installed so Palm's instructions were of no help. It turns out you can use the Bluetooth File Exchange utility to beam the two files over to your Treo and it will install them for you.
3. The JVM will let you install a Java app (midlet) but only by way of a URL. The URL you want is: http://www.gmail.com/app/v1.1.0/en/gm-Palm-Treo_650.jad (note that if I go to that URL on my computer I get a 404 not found but on my Treo it works just fine).

Hopefully this helps some people out. While you're at it, you should definitely grab Google's Mail app for the Treo - it's a PRC so the download 'just works', go to http://mobile.google.com/ on your Treo web browser (you better have unlimited data though, those map tiles eat up bandwidth quick).