Yeah neither are used in "proper" english, but I know a couple of instances where someone would say "I got you". Situation 1: Let's say you are going out to eat with friends but you don't have your wallet with you. You could turn to your friend and ask "You got me?" and if your friend wants to pay for you he/she would say I got you Situation 2: If you are helping someone off the floor and you tell them "I got you" meaning that you won't let them fall. Situation 3: You could also say I got you meaning like you understand what the other person is saying. Hope this helps!

Neither are very common but "I got you." You will more likely experience "I gotcha" before either of the two phrases you listed, same meaning. :) Also, when using quotation marks("") your punctuation goes inside them. Example: Incorrect: "I got you"? Correct: "I got you?"

This is an interesting question. I don't think in Britain we use either expression at all. Let's hear from native speakers from across the world.