pi-hole only resolves DNS queries for you. It not having a gigabit port would not make any difference as the packets it is sending/receiving are tiny and all your traffic does not go through it only the DNS queries.
Also my Raspberry Pi is currently set up with RetroPie but it only gets played once in a blue moon. Is it possible to set it up with PiHole for most of the time and then when I wish to play games, boot it into Retropie? Or would I need two seperate SD cards?
Actually two SD cards wouldn't be a problem because I recently upgraded from 16GB to 128GB for more games, so I do have a 16GB one kicking around somewhere I could use for PiHole, I guess.