If you're looking to manually set your gas fees, there are different ways to do so depending on whether you're in MetaMask Extension or in the Mobile app.
Regardless of the version of MetaMask you're using, keep this in mind:
The gas limit
The gas limit is the maximum amount of gas you are willing to spend in order to carry out a transaction or EVM operation.
The max priority fee
The max priority fee, also referred to as the "miner tip", goes to the miner or validator, and incentivizes them to prioritize your transaction. Most often, the value you put in for "max priority fee" will be the amount you pay.
The max fee
The max fee is the total, global amount paid for your transaction: base fee + priority fee.
In the Browser Extension:
(scroll down for Mobile)
Click on the Account icon:
Then, click on "Settings":
Then select "Advanced":
In "Advanced" please scroll down:
Then, enable Advanced gas controls:
The result should be the following:
Clicking on 'edit' should allow you to modify the three applicable fields as follows:
In the Mobile app:
When you get to the final screen before sending your transaction, you will see an "Estimated gas fee". To the right, the fee will appear underlined in blue:
Click on the fee, and a menu should appear with a slider, and another option to click on "Advanced Options". When you do, that menu will open and you will have the opportunity to customize your gas fully.