I used to use Evernote, which had a forward email feature. Using a special email address, I could forward an email from Gmail (web) into Evernote. How can I capture emails in DevonThink as needed? Is there a similar feature? Or a different way to do this?
No this is not possible. Evernote’s mechanism relied on them providing their own servers.
Capturing emails is covered in Help > Documentation > In & Out > Archiving Email.