First, we're going to change your profile picture in Gmail.
In the popup window paste in the URL you got from your Gravatar account.
Click on the Gear icon just underneath the area where you changed your profile photo.
Scroll down the window until you come to the signature box and delete the old logo image. Click the "Insert Image" icon in the taskbar.
Don't forget to save your changes.
In Google+, RIGHT CLICK on the profile photo and select "Open IMAGE in New Tab". (We're trying to find out what the URL is of your profile photo. You'll also note that when you changed your profile photo in Gmail, it also changed it in Google+
Copy the URL at the top of the browser window.
Paste the URL into the "Insert Image" pop-up window. You are using the Google URL rather than the Gravatar URL.
Click on your profile photo to open the profile information AS IF you were going to change your photo like you did before.
However, instead of clicking "Change Photo", RIGHT CLICK on the image itself and "Open LINK in New Tab".
When you do this you will be taken to Google+.
Go back to the Gmail signature area in "Settings" and click on the "Insert Image" icon again.
Don't forget to save your changes...