These two things are scary and usually the wallet sets them for you. But if you are in a hurry or want to pay as less fees as possible, you might want to edit them.
Maybe not all wallets allow you to edit Gas & Gwei, but I’m going to focus on two that do: Parity (Web) and TrustWallet
The Gas: The base unit is 21000 and this is enough for wallet to wallet transfers, no reason to set more. The smart contracts usually burn more than that (ex: you send tokens to Etherdelta or other trades/dapps). The values vary and Parity and most wallets will set the right one for you.
When you participate in an ICO, usually they recommend an amount that makes sure your transaction will go trough. Use that value to avoid running out of gas and loosing the transaction fee.
“Mistakes” can happen, as happened today with Insurepal ICO when they recommended via email 25000 Gas which was not enough for a smart contract, therefore most contributors lost the gas fees.
Gwei: Sometimes includes 9 zeros in the tail, sometimes not, depends on the wallet. Ignoring the 9 zeros, the values used these days are between 20–100, depending on the network congestion (real time data here). EthGasStation is a great tool to evaluate cost and duration. Few months ago, 1-4 used to be enough. If you transfer from wallet to wallet, what EthGasStation recommends should be fine. If you transact with EtherDelta (a very busy smart contract with concurrent transactions), you might need to increase above the recommendation (70–100 at this time). If you participate in an ICO, use what they recommend.
The fee you actually pay: This is determined by ETH price, GAS x Gwei factor. If you set a high gas for a wallet to wallet transaction, in the end will burn 21000 which is the actual cost, so the main factor is the Gwei value.
Example of a wallet to wallet transaction: Gas Limit: 21000, Gas Used By Txn: 21000, Gas Price: 0.000000055 Ether (55 Gwei). Actual Tx Cost/Fee: 0.001155 Ether ($1.30)
The amount transferred does not impact the fee or speed. You will pay the same fee for 0.1 ETH or 10.000 ETH (dreaming… :))
Conclusion:
- The Gas value: make sure is enough for what you do, at the end it will burn what it needs.
- The Gwei value: use recommended values if you are not in a hurry or not transacting with busy smart contracts. Use more if you want it to happen quick and accepting to pay higher fees.
Medium link: https://medium.com/cryptokytzu/about-ethereum-gas-and-gwei-b7c080701db2