[hunter2_]

To validate a signature, the code doing that validation needs direct access to the message prior to any rewriting. I don't think the proxy introduces any barriers to that access, assuming the validation occurs on Gmail servers, as the Gmail interface can present the results of that server-side validation.

If you wanted to validate it yourself instead of trusting Gmail to do it for you, you'd use the "Show original" feature which gives you the original (per its namesake) without any rewriting as well. I assume (but haven't tested) that connecting to your mailbox via IMAP, POP, etc. also causes you to retrieve the original, with the rewriting only coming into play when using the Gmail web interface.