Benko gambit is another that deserves studying by all active players. Many, many times you get similar positions in a game and utilize ideas you've learned in the Benko. I think the main line is 1.d4 Nf6 2.c4 c5 3.d5 b5 4.cxb5 a6 5.bxa6 g6
I believe there are some Benko-like positions that happened in this last Candidates tournament, but colors reversed (e.g. think some kind of c4 d4, b4 situation, maybe with e3 inserted either before or after). Super useful "gambit" that is barely even a gambit.
"Is there a reason why gambits have fallen out of fashion at top level?"
Yeah, there is: playing a gambit gifts the adversary with a sound strategical plan: accept it and try to keep the material surplus (and possibly the advantage) without falling in any traps. And if you misplay the game, you are already up in material, so it's easy to give something away and try to hold a draw. I basically described the style of Steinitz here (the first World Champion), and his successors are even better than him.
A case can be done for gambits in situations when there is little time, such as in internet games here on Lichess, or blitz and rapid OTB. You rely on the very same engine preparation to quicly navigate in uncharted territory for your opponent, and hope he makes a mistake. But in classical, no way a GM is going down withou a fight >>> that's exactly why in top level chess many try to emulate a "Carlsen approach": play an offbeat variation of a sound opening and slowly maneuver the resulting game in search of the advantage.
Generally a lot of gambits that historically did well have had antidotes worked out so fell out of favour. For example lines like the Vienna Gambit and Evans Gambit that were popular in the romantic era now have lines that at least draw for black. So anyone that has done prep for these lines that's expected for a 1...e5 player has an easy draw that isn't what white players want.
These lines are effective under 2000 as many players won't know lines well and will walk into opening traps but at GM and super GM level these lines lack venom.
There's also problem that now opening preparation and databases is a thing. So if I'm due to play an opponent who normally/often plays a certain gambit I can look up theory of the line so the surprise value is also lost.
If you like unusual opening lines then games of Michael Basman and Tony Miles will certainly be a source of inspiration although generally the main lines are the main lines for a reason.
It should also be noted that just cause a line is called Gambit or not doesn't mean players don't play it energetically. Top GMs will often sacrifice material for the initiative. So the important thing isn't whether you play a Gambit or not but instead whether you understand the compensation you get for the material and are able to play these positions correctly.
Gambits for white are heavily underestimated. However, finding sound ones for black that will score well and not just self-destruct is almost impossible (except the mainstream ones like the Benko).