Gambling is starting an attack without initiative, or "out of turn". Initiative is essentially having a timing advantage in which you will hit, and flinch the enemy first. You can 'get' initiative by starting an attack at the same exact time but with a faster weapon, ripostes, chambers etc. or just by starting an attack first.
If you're dueling (let's say with similar/identical weapons) and your opponent starts a windup, and halfway through that you decide it's a good time to throw an attack, the only possible outcome if both players commit to this attack is that you get hit. Your opponent has the initiative, and with this timing advantage they will hit you first every time. Gambling is ignoring that, and just throwing an attack out and literally "gambling" on the hope that the opponent feints, drags or does something to delay their attack and lose initiative.
It can be frustrating to fight against a gambler, since they just ignore the initiative or natural 'right of way', but once ...
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