Set up multiple signatures in Gmail - Canned Responses lab

-activate the Canned Responses in Settings->Labs
-click the Compose button to open a new message window
-Once your signature is ready, click the down arrow icon and then Canned Reponses>New canned response
-enter a name for your new canned response and click OK

Full instruction:

https://www.mail-signatures.com/articles/how-to-set-up-multiple-signatures-in-gmail/