A person who is your child in the future, but ends up as your brother in the present day.
John and Jane had a son. They went back in time and gave the baby to John's mother. Now the baby's John's brother and his son or his brother-son.
Short for Good People For Life; describes an awesome person who's the epitome of perfect.
Chris: did you see that awesome episode of Out of the Box?
John: yeah. Tony's totally a GPFL.