What is "GWei" setting?

GWei is used for sending transactions with specific gas price.

If GWei is 0, the gas multiplier value will be used otherwise, the DApp will ignore all the multipliers but use this GWei value.

So if you want to limit the GWeit to be more than 500 gwei because of gas spike, you can use this setting.

Also some contracts have GWei limit for buy / sell, so this is useful for that kind of contracts.