Mailplane is a probably the best way to use Gmail on a Mac. Version 3.6.9 adds native support for DEVONthink Pro and Pro Office: Open a message in Mailplane and choose Edit > Save to DEVONthink to save the message as webarchive to DEVONthink. The document includes a link back to the original message in Gmail. You can also send downloaded attachments to your DEVONthink inbox.
Click the app icon to have a look at Mailplane and check their release notes.