Gas fees are the costs of making a transaction on any blockchain. The value of the fees is dependent on the blockchain you are using and it's congestion at any given point of time.
How much gas should I use?
When making a transaction you can either use the fee that is suggested by default or adjust it to your own means. Depending on the kind of transaction you wish to conduct, you may or may not want to adjust your gas fee.
Setting your own gas fees
If you choose to set your fees, you need to set a certain gas limit for your transaction. A gas limit is the highest amount of gas you're willing to spend on the transaction. Smart contracts will normally estimate this for you. Higher gas limits facilitate more work on the transaction by miners. If the gas limit is not set high enough then a transaction will fail as the miner is unable to complete the requested actions with the amount of gas allocated.
Secondly, you need to set the actual gas fee. The gas fee is the amount you are willing to pay per unit of work (gwei) for a transaction to be processed. Miners are thus incentivized to choose transactions that offer more than the standard gas fees to process, thus resulting in higher fees for these miners. When considering how much gas you should use, you should take the complexity of your transaction into account.
The equation for how much in Ether you will pay in “network fees” is:
cost (ETH) = gasLimit × gasPrice × (1/10⁹)
Example — Alice sends 2ETH to Bob, choosing a gas price of 2 Gwei:
21000 × 2 × (1/10⁹) = 0.000042 ETH
More information on Ethereum gas fees and how you can most efficiently use this can be found here.
You can also find the current market gas prices here.