So, there's been an ongoing debate on these forums about when it is and isn't acceptable to say "good game" (or "gg") to your opponent
I usually use it [...] regardless of [...] how open the game still is
Some people don't care at all about when and how you use "gg", but among those who do there's a clear consensus that saying "gg" while the game is still open is terrible. Either wait till the game is actually over or you intend to resign right after saying it.
Edit: obviously if you just want to express what a cool game it is you can say
that at any time during the game, but don't use "gg" for this. "gg" has become more like the handshake you give your opponent at the end of a chess game, you don't do that halfway and then just play on.