Gmail ‘Show Trimmed Content’ – Keyboard Shortcut

To label the currently open mail, press L and start typing the label. Matching labels will appear as you type, so you can just navigate to the one you want (using arrow keys) and press Enter.

To label AND archive the mail at the same time, press V instead of L and follow the same procedure.

In the mail list view, use J and K to move up and down the list (the tiny arrow to the left of each item will move), press X to select one or more mails and then use L or V to add labels.

Google has heard you and there is now a hyperlink shortcut in Gmail:

Ctrl + K (Cmd + K for Mac users)

It works only if the cursor is in the message field, not in the To, Cc, Bcc or Subject ones. And, in Chrome on Mac, it works even when the shortcuts are disabled.

See here.

This shortcut is not yet documented in the official Gmail keyboard shortcuts list, so don't look for it there.