I finally downloaded and tried GNU Go. Wow! I hope others will post adulations, but I’m so astonished I’m speechless!
I do have two minor procedural questions. (I’m embarrassed to ask these nitpicky details when the program is otherwise so very impressive.) Is there a way to continue a match after pressing End Game? (This is a minor nit, as I don’t mind just pressing New.) A more bothersome problem is the keystrokes Ctrl-R and Ctrl-F which may be required every roll. Can I replace them with just R and F? Or should I just lay a brick on the Ctrl button when running Go? 
Even making them the same keystroke would be much less finger effort for clumsy me. (Ctrl-F is never legal when Ctrl-R is, right?)