Hello I'm new to Gwent,
Wondering how the feature on a card "Brave" supposed to work?
Example sometimes the neutral gold card "Geralt" activates it and turns into 13 card as oppose to 10 card.

How does this work and why?

It work if you behind in your score after playing the card.

Say you have a 10 strength gap in favor of your opponent, playing Geralt will not trigger the ability, however a 11 strength gap will.

To this i only want to add that "Strengthen" raises base strength, while "Boost" is only temporary and can be reset by lots of things.

When an ability is "brave" it occurs only when you are losing. For example, Geralts's one happens on "Deploy, brave". So let's say your opponent has 23 total strength and you have 12. You play Geralt so your score goes up to 22. Geralt's "deploy" power triggers because you just deployed him - but it's also "brave" so it first checks your score to see if you're still losing, sees that you are, so Geralt gets strengthened by another 3, putting you at 25.

"Strengthen" means the card's strength increases. It's almost the same thing as "Boost" except that "Strengthen" increases the base strength whereas "boost" raises the strength without changing the base strength.