A gee-gee is a horse — but only in the kind of English you use with small children, or when you are being playfully babyish. A little child “goes to see the gee-gees”. An adult talking seriously to another adult would just say horse. It is mainly British; American children are more likely to say horsie. The feeling is warm and a bit silly, never a textbook word. In older British talk, the gee-gees could even mean horse racing or a bet on horses, though that already sounds dated. You would not write it in a science report, and you would not use it about a wild horse in a nature film unless you were talking to a child.