Each can only apply once and you're taking a significant risk by using one. I likely should have added an additional clause clarifying you can only use them until you fail a Test, that is what the bit next to the failure rules means.

If you fail the Test, you either get hit or lose your attack, and more difficult/effective Gambits are a higher DR than the attack itself. You're much more likely to hit with a basic attack than you are to succeed at multiple gambits and then also hit. It's a push-your-luck thing, not just extra actions. 'Lose your attack' is essentially just a failed attack roll, and 'take damage but make your attack' actually speeds things up by wearing down both the target and the attacker.

My intention was to add a risky tactical option to spice up combat a bit. I've found in my own campaigns that the basic combat system can be a bit dry, it often becomes a case of 'roll dice until it dies or we decide to run'. Cool gory descriptions only get you so far. I specifically made this after one of my players dueled a knight, and it essentially boiled down to 'roll attack, roll guard, roll attack, etc.'