Here's something I searched up in two seconds:
"The king's gambit is a good opening when played in lower-rated tournaments . . . it is however not a good choice in mid-tier/high-tier competitions since it is solved to be better for black."
This is a pretty bad explanation. I searched (briefly) for the author's rating (Michael Stephen Vargas). I didn't find it, but even if he were rated as high as something like CM, it would just mean he was much worse at teaching chess than playing it.
It's certainly a sub-par choice if you're in a situation where people know you, and are preparing for you. Not because it's "solved" (poor word choice) "to be better for black" but because black has many good options, and as white you have to learn them all.
Other than the upkeep cost (you'll have to refresh the massive required theory now and then) black also has the choice on the type of game. Black can choose wild tactics, or calm positions. White does not get to choose.
And for "high-tier" tournaments (a term he needs to define) it's a problem if black is the one choosing whether or not there will be winning chances, and whether or not they want to equalize.
So it's a poor choice for 3 reasons, none of them having to do what the engine's evaluation or being "solved" (lol).
@Optimissed - I'm sorry that a database of over 377 million games played does not abide by your preconceptions. Apparently those 377 millions games don't conform to how people actually play chess as opposed to those theoretical books written by masters. Everyone knows that the conventional wisdom is always right.