I remember reading in a thread that Robert Fischer founded a tactic that made the Kings' Gambit look silly...
King's Gambit



I not saying that it is unaplayable but it is no longer seen at a high level


see Qxe8's comments and add the fact that "old players" played for different reasons. They played mostly as amateurs, for fun, not as a career, so losing and winning didn't carry such dire consequences. Also add that opening theory has grown exponentially since the Romantic Era. Most time is spend studying the intricacies of less explosive systems. Open games, as a rule, as seen less at high levels since one misstep in an open game is usually all it takes, while in closed games there's generally more room for error.
I've always liked this, from a lecture by David Bronstein:
"Bronstein played the first two moves of the King's gambit on the display board. 'This is what you should play,' he said. 'This is what you can learn about chess. Play for fun. Play the King's gambit.' As for openings popular with grandmasters, he said, 'They're boring. Look at this. Nowadays they play something like this.' He played out the first three or four moves of a Nimzo-Indian defense and an English opening. 'This is popular with the grandmasters. But it's boring.'"


I played the KIngs Gambit extensivly during the summer and had great results. Black usually hasnt done his homework or settles to lay the Falkbeer counter gambit which i found enjoyable to play as white. The Kings Gambit for me put weaker players to rest quickly while because i knew the theory well i always had good games with stronger opponents.
I wouldnt say the Kings Gambit is flawed in the sense that its unplayable savy_swede.

According to Robertie's "Winning Chess openings", "the king's gambit is an excellent choice for players who like unbalanced, wide-open positions with dangerous attacking chances for both sides..." I've been practicing this as a beginner/intermediate and I like the action of attacks and tactics rather than more closed, slower games that require "strategy and defense"-- which is more for advanced players.
I heard that the King's Gambit used to be the most popular opening in the old days. What flaws made it fall out of favor?