I'm trying to @mention a co-worker in Gmail using TB, but it does not expand the @ to a link and add them as a recipient. It simply sends the @ as text.
Also, I setup @mention as a dropdown of different co-worker names and allow for multiple select.
We have a special command pack for Gmail to support that kind of scenarios. You can find instructions and additional information about command packs here.
Once you have the command pack you will be able to add an email to cc with {gmail-cc: susy@example.com} or to bcc with {gmail-bcc: greg@example.com}. This will not however add @mention as a text, so if you still want that I suggest using @mention manually once let gmail replace that with a link, copy that text with link and paste it in your snippet.
As an alternative you can use autopilot commands to simulate actual typing and type @mention. For example {key:@}{key:m}{key:a}{key:n}{key:s}{key:u}{key:r}{key:enter}. This will simulate typing @mansur and pressing enter.