i have a similar thread before. my 2 gks were getting like 4-5 rating. i thot this was normal as i'm winning most games, and i had few losses. i sold one of them. bought a 4 star gk. he averaged 7 rating in about 15 games. unfortunately, i sold him also and the other gk. i got 2 5* gks. so far in total of 6 games, both are averaging 7 rating.

so to answer your question, just sell your gk. there are good ones out there in auction. once you find a good gk, use rest packs to prolong their usefulness.