Gambits might seem confusing at first, but really all they are are orders for your NPCs (i.e. non-playing-characters, the ones you're not controlling yourself). So let's say bunny girl is in your party. You could have her gambit list read:

1. heal any party member damaged to less than 70% of their health
2. fight nearest monster

So if everybody is healthy, bunny girl will keep fighting nearby monsters. If someone DOES get hurt, the bunny girl's first priority is to heal that person. Once that person is healed up to 70%, she will go back to fighting enemies.

You can add more gambit "spots" as you go. So while bunny girl starts with only 2 things she can do, you can soon have her doing 3, 4, 5 or more things based on precedence. You add more gambits in the license board, in the top (spell) area.

You can also find more gambit conditions. For example, in the mines right before you run into the bounty hunters you find a gambit to do something to enemies that have > 450 health. So this might be a way to prioritize attacking big monsters before little monsters.

You don't learn about gambits until you run into bunny girl and her friend. Once you do, you can enable and disable gambits. Remember to disable gambits if you're trying to run away - or at least change them so the NPCs aren't trying to engage random enemies while you run!

Final Fantasy XII Walkthrough