By default Gmail doesn’t allow HTML signature in outgoing emails. Means all tags you put into signature will appear as it is.
There are multiple ways to overcome this problem. Simplest is too have a HTML editor and copying HTML signature into Gmail, thanks to Gmail’s WYSIWYG editor. This method is good for one or two mails everyday.
Firefox users can install Gmail – Insert HTML Signature script.
After installation, script will create a HTML signature box in left sidebar. There you can put as many signature as you want and also set which signature to be used by default (in case of multiple signature).
Also each signature can be assigned a memorable name as shown in screenshot below which appears when you add new or edit existing signatures!
Once done, every time you compose a new email as well as reply to any, your signature will be be appended automatically.
if you are using plain-text signature in Gmail, that will be also added to new mails. You can either use both or remove old signature by going to Settings >> General >> Signature!
Related: 3 Free Email Signature Generators