Check GMail with IMAP and Mail.app
GMail, when accessed via IMAP tends to do some "interesting" things. There are several settings that lessen the pain:
1 - Click on, 'Settings,' at the top of the GMail web page while you're signed in. Click on "Forwarding and POP/IMAP" and then scroll down to the "IMAP" section.
2 - Update the following settings:
"When I mark a message in IMAP as deleted" - Auto-Expunge off
"When a message is marked as deleted and expunged from the last visible IMAP folder" - Immediately delete the message forever
"Folder Size Limits" - leave as default
3 - While still in settings, click on "Labels" and then *uncheck* "Show in IMAP" for the "All Mail" label.
That last step is really the most critical as Mail.app generally can't keep up with the amount of mail that shows up in "All Mail" and needs to reindex on each and every mail check.
Hope that helps people checking their GMail via IMAP and Mail.app.