What is Gas on Ethereum blockchain

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Gwei is just a smaller denomination of Ether, at 1X10^-9 ETH. A wei is the smallest unit of Ethereum you can have, at 1x10^-18 ETH. Ether is used to purchase Gas, which is required to run smart contracts and submit transactions to Ethereum. Gas is the primary method through which miners processing smart contracts get paid. Gas is used to ensure that even though the price of Ether may fluctuate, miners will always have a standard from which to measure the processing work they do. When you prepare a transaction, you also send a "bid" on how much Ether you are willing to pay for the Gas costs to process your smart contract or transaction. Your Ethereum account is charged directly for the gas used in processing your request at the rate that you bid (same as a traditional 'transaction fee', so, for example, sending 5 ETH might cost a total of 5.0001 ETH). So your 200,000 gas limit really doesn't have a constant ETH value, it depends on your bid.

A tx will have a fixed cost in gas, gas_cost. The cost in ether = gas_cost * gas_price. Gas_limit is the maximum amount of gas you'll provide. Gas_price is the ether you'd pay for each unit of gas. Gas limit is there to protect you from buggy code running until your funds are depleted. The product of gasPrice and gas represents the maximum amount of Wei that you are willing to pay for executing the transaction. What you specify as gasPrice is used by miners to rank transactions for inclusion in the blockchain. It is the price in Wei of one unit of gas, in which VM operations are priced.