The King's Gambit isn't a bad opening; it just requires a lot of opening knowledge because of the compicated lines and sacrifices, so it might not be suitable for a beginner. On the GM level, it isn't played very often becuase black can usually obtain equality relatively easily.
For a similar opening against 1.d4, maybe the Modern Benoni would be a good option. It's a very tactical, double-edged opening where black can obtain some good counterplay if he/she knows her plans. Unfortunately, it also requires a lot of memorization to play the Benoni because of its risky nature (although I'm assuming this wouldn't be an issue for you as you're playing the very complicated Sicilian).
A typical start for the Modern Benoni. Black will develop his bishop to g7 and 0-0, with the plan of queenside play with a6 and b5. White on the other hand will go for the e4-e5 pawn break and try to attack on the kingside.
I've read that open openings are more suitable for novice players, I agree with that since, by my experience, they lead to simpler games were, in general, at which point of the game there is an principal issue that is being disputed between the players. In closed game the way to play is through maneuvering, a kind of cold war, and is often difficult for the beginner to grasp the reasons behind the moves. The open games are more frank and is easier to understand the meaning of each act.
Based on that I choose to always open with 1.e4 and as black to answer that with 1... e5, in general leading to Ruy Lopez. I recently changed to Sicilian 1... c5 and I'm very happy with that since I feel this leads to a much more straight game where, to win (as black or white), I can choose to engage on an attack or go to the endgame with a subtle advantage that can be converted into a victory.
Recently I'm interested to also not engage in Ruy Lopez also as a white, so after 1.e4 e5 I'm trying to answer with 2.f4, the King's gambit opening. I like to be aggressive but I really don't feel comfortable in this opening, I feel there's a too big sense of urgency, timing is essential, too much for a novice, because our moves are not always so precise.
So I would like to ask if the King's gambit is really a bad opening, in the sense that if both players have the same level and the same understanding about this opening then black has true advantage. If both players have the same level then white has an advantage only if it has a bigger knowledge about the King's gambit?
I feel that this opening is not good if your level is too low (by the above explained reasons) or if your level is too high since it is a too rushed opening (indeed it is almost never used in GM level). There is a level where it can lead to a fair dispute?
I also would like to ask, in the same spirit of open openings for beginners, what would be a good response to 1.d4.