The problem is the same one described on this thread on Google's help forums, requesting that Google recognize the Touchpad as a tablet device for the purposes of Gmail (the thread was started by some Redditors I believe).
Can we get the iPad-optimized Gmail interface on the HP Touchpad? - Gmail Help
In my research the only solution I have found is in the following thread, which advocates editing your WebOS user-agent string. This method has it's own problems, specifically that ALL sites will load their mobile versions, when that's not necessarily desired.
I've also tried getting the URL to the tablet version, which you can discover by editing your Chrome user-agent string to be an iPad and then noting the URL that Gmail takes you to. However, when loading this URL directly on a Touchpad, Google shows a 404 error.
I think the ideal solutions, pending any intervention from Google (which I don't have my hopes up for) are:
1) A "patch" that selectively changes the user-agent string to be like an iPad *only* on the mail.google.com domain.
2) An "app" that is just a copy of the main browser but just loads Gmail and uses the iPad user-agent string.
Does anyone know how to do this?