if i send some eth from coinbase to a metamask adress do i pay gas fee?
Yes
if i swap some token on uni i pay gas, ok but is it fixed or a percentage of the amount?
Its depending on the gas price at that time
gwei means dollars or is a fraction of eth?
U can read here :
https://www.investopedia.com/terms/g/gwei-ethereum.asplast question, if i want to remove my authorization to a smart contract to have unlimited acces to my wallet do i pay gas?
Nope, u can use
https://app.unrekt.net/