IANAL, but it sounds like you can use the gopher in both cases. After you add it as your G+ profile picture, go to your "Photos" tab and into the Profile Pictures album and click on the image, then in the lightbox you should be able to add a caption with the appropriate attribution ("Go Gopher by Renée French"). For your issue tracker, you can probably just add it to the footer (""Go Gopher logo created by Renée French") or possibly as the HTML title of the image so that hovering over it shows the attribution.
I could, of course, be wrong.
~K