Generally it's best to work out titles with the child's parents. Some people feel very strongly about their title and choose it themselves. For example a grandmother might be Nana, Granny, Grandma etc. Others, especially nonrelatives, don't mind what they are called and are happy to let the parents choose or go with a default "Auntie" or "Uncle" often given to close family friends. So ask yourself, if the child did visit you, and wanted your attention, how would you want to be greeted? "Unca Bob"? Just plain "Bob"?
Then check with your friends, since this hasn't come up before, that they are ok with your choice. Many parents don't like their children calling adults by unadorned first names, for example. I've been overruled in my "call me Kate" wishes by parents who insist on Mrs Gregory or Auntie Kate. Since these friends of yours are close enough to name you a godparent, you're close enough to have a quick chat about their preferences as well as yours.
Finally, when it comes time to sign the card, you don't need a single title to do all the heavy lifting of reminding him who you are. You can write something like
Happy Birthday from your doting godparent, Uncle Bob. Looking forward to playing more [whatever system your handle is on] together. Remember [Child's Handle] doesn't always beat [Your Handle] :-)
Now you've solved the problem of writing something more than just Happy Birthday in the card (I always felt, with the nieces and nephews I didn't see often, that I really didn't know what to write), asserted your godparentness, reminded everyone how you want to be greeted, and helped connect this card to the person the child plays with online. Mission Accomplished.