Every transaction in Ethereum network requires a resource called Gas to be executed. Amount of Gas corresponds to the complexity of a transaction. If you set Gas limit too low, your transaction will fail. Think of it just like having gas in your vehicle (ETH in your wallet), and the destination is the ICO address. If you don’t put enough gas in your vehicle to get to the destination you’re going to, you won’t make it. Lower Gas setting will take longer such as driving a fuel efficient vehicle at slower speed.
The units of gas necessary for a transaction are already defined by how much code is executed on the blockchain. If you do not want to spend as much on gas, lowering the gas limit won’t help much. You must include enough gas to cover the computational resources you use or your transaction will fail due to an ‘Out of Gas’ error.
All unused gas never leaves your wallet. So if you go to your DECAMASK Wallet, send 1 ETH to DECA’s smart contract address, and use a gas limit of 100,000, you will receive 100,000 - 93,377* = 6623 back. However, if you were sending 1 ETH to a contract, and your transaction to the contract fails, you will use the entire 100,000 and receive nothing back.
*93,37793,377 is the standard DECA’s gas limit for regular transactions.
If you want to spend less on a transaction, you can do so by lowering the amount you pay per unit of gas. The price you pay for each unit increases or decreases how quickly your transaction will be mined. 
For more information please check testnet (ropsten)Transaction at, where we give 0.5 ethereum to DECA’s Smart Contract: