Using OnLetterhead with Gmail's web-based email is a brand new feature! Using OnLetterhead with Gmail is a very easy process, however Gmail's editor must be in the Rich Text mode to successfully display the OnLetterhead designs. The good news is you probably don't have to change anything to use OnLetterhead with your Gmail.
Your Gmail toolbar should look like this:
If you see this toolbar when you click the Compose button, then you are good to go.
You may see this when you click the compose button:
If so, please click the Rich Formatting button to switch to the Rich Formatting toolbar.