Best opening against white's King's Gambit?

Hello again. Could anyone help me out please. I am trying to figure out a way to find an opening that would help me refute the King's Gambitas black. I am having increasing trouble with this opening. I would be very appreciative of any assitance that could be given. Thank you.

Have you tried 2...bc5 ?

There's a lot of great replies, the best that's been proven is the Fischer's Defense against 3.Nf3 and 3...Nc6 against 3.Bc4.  For 3...Nc6 it's best to know the ideas and reasons behind the moves or else you're playing an empty refutation (lose the thread, equalize, and white may even come out on top!) 

As for 3.Nc3 it may be tempting to displace the king with 3...Qh4+ 4.Ke2 but that's exactly what white wants and prepares for. He'll probably prepare against 3...Ne7 too but this is recommended by me and was good enough to draw Carlsen by a 2500 in a blitz game. 

To my mind, Fischer defense (2.f4 ef 3.Nf3) d6 is unpleasant for White (I am very fond of King's Gambit as White), and in case White plays something else instead of 3.Nf3 - 3.Bc4 Qh4+ 4.Kf1 Nf6 (4. ... d6 is also good), or sharp 3.Nc3 Qh4+ (3. ... Nc6 is also good) 4.Ke2 Nf6 5.Nf3 Qh6

Keep in mind that with 3...Qh4+ against both 3.Nc3 and 3.Bc4 you're walking into white's home prep.  John Shaw recommends 3...Nc6! against 3.Bc4. 

Thank you so much everyone for your comments. Thank you also for those annotated games. They are serving me well in studying them. I'll let you know if I get any results soon.

You are quite right, for Black it's easy to get into some "home made" (or better to say "home prepared") unpleasant trick, especially in the lines where White allows Qd8-h4+ (although in many cases that queen move can be in reality Black's most unpleasant reaction for White) and it may seem that Black gets significant advantage there.

Just one thing for Black - prepare your own "home made" tricks, too, to be at least in more or less equal condition as your well-prepared opponent

The king's gambit cannot be refuted!

I usually play 1.e4 e5 2.f4 Nc6 3.Nf3 f5 - not for the faint of heart. You are going to get a lot of detractors saying it's a losing move, but at club level it's viable. The only problem you have to face when playing these offbeat lines is whether or not your opponent knows the refutation right there at the board. What are the chances?

I play 2...d5 even though I don't know the theory. The game is open and you just play.

Just curious, what do you play upon 2.f4 d5 3.ed ?

3...exf4. There are other moves but they since they're gambits they probably do require you to know some theory.

As White, I never faced 1.e4 e5 2.f4 Nc6 3.Nf3 f5, @cornbeefhashvili. And it seems quite a good way for Black. But I did face several times 1.e4 e5 2.f4 f5  Your idea of 3.Nf3 f5 is better than 2. ... f5

declined

Although 3.(ed) exf4 also leads to the theoretical positions - it is the best among the other moves played there, even if 3.(ed) e4 is the most popular. I suppose in most games where you play 3.ed exf4 your opponents probably go on with 4.Nf3, what do you play then, if I may ask you?

Just a question but is that the Grand Prix Attack when you play 2.f4 or is it called the King's Gambit? I know it is a silly question but I need to know.

Grand Prix Attack vs. the Sicilian.  It leads to a very different game from the King's Gambit.  Against the Grand Prix you want to play 2...d5! 3.exd5,Nf6 4.c4,e6 5.dxe6,Bxe6 and you have amazing development and control over d4.  It isn't a forced win for black, but white's position is full of holes. 

If you mean 1.e4 e5 2.f4 it's King's Gambit for sure. I have heard several times about Grand Prix Attack, but I have no idea what that attack is. Rather often I come across some strange names of variations - strange to me, but as it afterwards turns out, I know the same variations but under some different names

King's Gambit Declined is usually what I use. If I'm playing a n00b, however, I will accept in hope that they will play d4 instead of bringing out the knight, which allows for havoc-wreaking on the queenside.

I like the fischer defense.