I fk means I Fucking Know
Person #1- Dude, you need to pay me back
Person #2- I fk
56gay is the equivilent to 56k. You use it to symbolize that someones internet sucks.
Me: "I just downloaded this game demo you should download it"
Friend: "I can't I only have 56k it would take forever"
Me: " haha you have 56gay"