A few comments regarding K-9 Mail.
First of all, I use it for ALL of my POP email accounts and I really like the app. Not perfect, but far better than the stock Gmail and Email apps.
Second, I always update using their
DOWNLOADS page. They seem to lag far behind with their Market updates. Although, probably rightfully so as a lot of their updates are beta versions.
Lastly, you may want to play with using either POP or IMAP settings for your Gmail account. If you simply setup Gmail when you first open K-9, it defaults to an IMAP account. You can change this later on manually.