Questions about game mechanics

I’ve been playing the game for about 30h.

And I’m approaching the end game content. My character is a warrior who uses his shield to block and do damage.

But how does exactly the % damage on block work.

It’s a % of what?

How is it calculated, and how can i increase it?

My second question is how do exactly the gems work?

If I put one in for example a helmet does the bonus transfer to the rest of their equipment, or is it only for that piece of equipment?

Doors for example a stat that says 40% power yes yes The only gem. So does it work if I have other gems in other equipment parts?

Thanks for the help.