well, you're lucky he wasn't a great player and that he gave you a knight for free!
you have to either learn a good king's gambit defense (like the fischer or modern) or a counter gambit like d5. unfortuntaly you can't just play the king's gambit "naturally" by making normal moves or you end up badly.
It looked like white was kind of confused by the fact that you didnt take the pawn. there are a few places where different moves could have been played to gain a better position, but in agreement with TraglorfBob, most of the mistakes were made by white.
this is a normal king's gambit declined, i would usually accept the gambit though.
i honestly cant find any errors in your side of the game.
the only thing is that not accepting the gambit pawn leads to white having the f-file half-open.
There's a lesson here on dealing with the king's gambit as Black:
https://www.chess.com/lessons/every-gambit-refuted/every-gambit-refuted-kings-gambit
To refute a gambit, accept it.
The King's Gambit has never been refuted, not even in computer chess championships. It's dangerous below top level play, but can still be dangerous there. Judith Polgar beat Vaselin Topalov in 26 moves the same year he played Anand for the world championship.
I play the KG all the time and players react differently every game. I'd say the Fischer Defense is the hardest to play against because rather than trying to uselessly attack the "open” king, it plays solidly and says "what'cha got, you're just down a pawn.
I have only been playing chess on a daily basis for a few months now, so I am a pretty weak player. The following game was my first encounter with the King's Gambit. I was black and I declined the Gambit because I value position much more than material and one of my biggest peeves is stacking pawns. I was wondering if you all could give me some suggestions on how I may have dealt with the situation more effectively. Thanks for the feedback!
If you can think of any suggestions outside of the gambit opening as well, please feel free to respond. Thanks again.