GTFK is an Acronym meaning "Good To Fucking Know."
Generally used to express sarcasm after having received unsolicited advice or information, GTFK can also be used to express sincere appreciation and/or genuine enlightenment.
1) No shit, Einstein! I'm so glad you decided to tell me, that's certainly GTFK!
2) I'm so glad you told me, that's definitely GTFK! You've saved me countless hours of wasted work.