Gmail does not offer multiple signatures like Microsoft Exchange did. There are a few work arounds that can make it work, though.
Log into your Gmail Account online.
Select "Enable" for the "Canned Responses" lab.
Select Save Changes at the top or bottom of the page.
To add your signature to a message:
1. Type your message
2. Place the cursor where you would like to add your signature
3. Select Canned Responses underneath the Subject line.
4. Select the Signature you would like to add.