What is the Best Gambit or Strategy?
@1
Most gambits are unsound. Queen's Gambit is the best gambit. Catalan, Two Knights Defence, Marshall Attack are considered sound too. Benkö Gambit, Evans Gambit, Smith Morra Gambit are borderline sound.
@5
Yes, Catalan is a gambit. 6...dxc4 wins the pawn.
https://www.chessgames.com/perl/chessgame?gid=2122755
Yes, Two Knights Defence is a gambit, Bronstein called it Chigorin Countergambit
1 e4 e5 2 Nf3 Nc6 3 Bc4 Nf6 4 Ng5 d5 5 exd5 Na5 loses a pawn by force.