Thanks for trying out my game! (and for liking the music <3)
The AI is very very simple, as it never managed to get anywhere complex in my short time of building the game. However, why it sometimes seems worse than other is because, when you press play, it chooses one AI from a list of ones I chose (so it isn't always the same one). Personally, all AI are bad in their own ways I think xD
Also, the reason the movement is so janky (away from centre, circling around, etc.) is such that the AI is rotation-invariant: whether you are down and he is up, or you are left and he is right, he will respond in similar fashion! Makes him way better than otherwise, at the cost of difficult controls...
Now, for GP, it is a difficult topic. It is hard to find stuff online for it. Personally, I learned it through my College, as I studied AI, so I have slides for it (in Dutch though, not too useful perhaps). The idea is from Genetic Algorithms (GA), of which there is pleeeeenty of information online. Now, instead of 'bitstrings' as representation, you use 'Lisp structures' as it is sometimes referred as. It is a bit too complicated to go in-depth here, but maybe a free online book like 'Clever Algorithms' by Jason Brownlee and such can help you out if you're interested (chapter 3.3).
However, I believe it is generally considered a not too useful AI technique, as Neural Networks outclass it in every way possible. I still love GP though, so I wanted to make it at least once in my life :D