Second, we are strictly speaking about the King's Gambit Accepted. That is, white begins with e4 (King's pawn forward two spaces), black replies classically with e5 (King's pawn up two spaces), white then gambits the f-pawn (King's bishop's pawn up two spaces), and black captures the f-pawn, accepting the gambit. As TFA mentions, the King's Gambit Declined has not been examined nearly as thoroughly.
Third, all of this is only somewhat relevant to actual chess playing, and only at the very highest levels of play; the average FIDE Master (i.e. a well above average tournament player, though nowhere near being among the 1,000 best players in the world) need not remove the King's Gambit from his repertoire because it has been "solved". This has, historically, been one of the most dynamic openings in chess, with tons of opportunities for tactical tomfoolery and psychological pressure. When we talk about "perfect play", or "near perfect play", we're already reaching beyond the level of world champions.
Fourth, while not every line has been thoroughly analysed, the ones that haven't are irrelevant. An advantage, in chess, is calculated on the basis of a difference of pawns. So, if the black player has all the same pieces as his opponent, save for an extra pawn, all other things being equal, we evaluate the position as -1 (i.e. from the perspective of white, the position is minus one pawn). Pieces other than pawns are weighed differently, even when we are solely looking at material differences. Traditionally, knights/bishops are said to be worth three pawns, rooks are worth five pawns, and the queen is worth nine pawns. However, the actual position of the pieces affects their worth; a knight very near the centre of the board is, often, worth more than a rook (i.e. A knight near the centre can have up to eight possible moves, whereas a knight in a corner can only have two possible moves). Thus, a position that has been evaluated as +/- 5.12 means that one player has more than a rook's worth of advantages over his opponent. Even in low level tournament play, it is very reasonable to assume that the advantaged player will win the game; at grandmaster level, this is so certain that it is considered impolite, even downright offensive, if the disadvantaged player refuses to resign.
Fifth, while different computer chess engines do evaluate positions differently, I have yet to come across a position about which the analyses of different engines have diverged by more than 2 pawns. An evaluation of +/- 5.12 by a top-notch engine can safely be assumed to be conclusive, since since most of what I said in the above paragraph also applies to an evaluation of +/- 3.0. Whatever else it may be, Rybka is certainly a top-notch engine.
Finally, it is true that Rybka's having reached its current strength relies on what are at best described as questionable appropriations of others' source code and algorithms. Nonetheless, the presented findings have an intrinsic value that is not dependent or reliant on notions of intellectual property or publicity. I am frankly ashamed by posters who have suggested that this article ought not have been publicized by slashdot because of its source. Knowledge is knowledge, period, and while it is both sensible and necessary to place ethical restrictions on scientific methodology, it is simply insane to deprive oneself and others of data that has, for better or worse, already been gathered.