I was actually trying to do this just last night too. My gf just got an Epic and she had 202 unread messages. For the life of me I couldn't figure out how so I did some research and apparently the gmail app doesn't have a select all or even a mark all as read option. The only way we could do it was through the browser gmail on my laptop since I couldn't figure out how to select all through the mobile browser gmail either. I wish the gmail app had these options since I actually need it as well. Does anyone know of another email app on market that has these features?
There is currently no way to select all messages. You will have to get that little checkmark next to the ones you wish to delete. As an alternative you can specify on your Gmail application how many of the last days of mail you want synced. Open the Gmail application and go to Settings>Labels. Here you can set the Gmail application to only sync maybe the last two days instead of the default four.
Yes. K9mail does have this feature. I use it for my imap and pop3 accounts but like keeping gmail with google. They really need to add this option. That and the option to delete the trash.
I guess as a workaround, you can login into your account at gmail.com through a browser, archive your mail there, and it should move out of the main mail folder on your phone when it syncs.