Is there "select all" functionality in the GMail app?
I'm not sure exactly how you use the select all feature, but I think an effective way to mitigate your need for this is to go crazy with filters, labels, and the priority inbox.

Having your Gmail account sort your E-mail into highly specific labels for you and carefully tagging everything as either "important" or "not important" will keep the number of E-mails you see at any one time to a reasonable amount most of the time.

There are a ton of great tips in this article from LifeHacker that I've used both on the web interface and on my Gmail app.

Oh, and submit a feature request to Google. ;-)