Beginner question about Gas Price

I'm still new to this, and about to sell my first NFT. I've read on every transaction or mint I would have to pay for gas. Is the gas paid with money or ETH? Meaning should I get some ETH into my wallet before I start selling?

Also people say to do the transaction when the gas is low, but I'm not sure what low means, or how to look at the stats (https://www.ethgasstation.info/):

2| STANDARD < 5M

$0.11 / Transfer

Gas Price (legacy): 27

Can someone explain, thank you.