I thought eth and gas are different? What's the right decimal value of eth called? as bitcoin is satoshi
I believe it is called a gwei... you're also able to set how much gas you want to use in a transaction, more gas = faster transaction processing. Most exchanges set gas at 4 , some let you set your own gas limit.
If you're sending an ERC20 token yourself (i.e., not going through an exchange) you have to have some ether (a tiny amount) to be used as gas. Some exchanges such as etherdelta convert and use other ERC20 tokens as gas in place of ether. For example, if you have a ERC20 token you're moving out of etherdelta, then etherdelta will take a gas equivalent amount of that ERC20 token for themselves instead of ether.