I assume you're talking blitz games here. You don't really need to know King's Gambit. Better to perfect things like tactics. Thing is, King's Gambit requires a bit of opening knowledge as white.
If you want something simple yet aggressive, try the Vienna (2. Nc3, 3. Bc4 etc.) . You get opposite castle positions a lot and can then attack. Just hope that your opponent doesn't know a simplifying trick at the start.
Basically you are aming for fast develpment and a quick crash on f7. Easier said than done. And at your level, ANY opening is effective. Hell, i regularly beat 1500s with the spike (also known as the Grob),so, don't worry if it's still "effective". At your level, the 3 knights is "effective".
Correction: The Spike is not the Grob, but rather a variation in the Grob where white plays g5 at some point after g4.
I know that. But, his opponents probably don't. If they don't the attack is his. If they do, he can simplify to an ending if he's good at endings. The point is to get an attack from Qh5 typically requires a gambit. He should learn to attack without a gambit.
You'll have to decide for yourself if it's effective or not FOR YOU! You can do so by playing it as much as possible with White and getting friends, acquaintances and even total strangers (if possible) to play it as White and see what defenses work for you as Black, and which ones don't. Playing any opening you're interested in from BOTH sides of the board is very useful - and it might come in handy sometime.
You can look up articles on KGA/KGD here, at The Chess Cafe, via Google search engine and look for used books (unless you're rolling in dough) on KG at amazon, eBay and USCF.
http://www.chesscafe.com/archives/archives.htm
basically you could defecate in a bucket and it would still be 'effective' at your level , AT YOUR LEVEL
All basic theory you need to know is 3.Nf3 if your opponent does not paly the Falkbeer and, if you face the Falkbeer, pray for luck and move exd5.
In case you do not find the Falkbeer, follow RoseQueens' advise (develop quickly and attack f7). If you face someone who knows the Falkbeer.... the first time you are going to be wiped out.
I love playing that to frustrate King's Gambit players.
That "trick" leads to the richest positions in all of chess, so far as this cowboy's concerned.
But 3...Nc6, 4...Na5 leads to a quick = (and that "=" is probably the best case scenario for white), and not particularly exciting play.
Would someone please be able to offer any advice on playing the King's Gambit? Any theory about how to play it or opinions on whether it is still an effective opening would be very helpful, thankyou.