Hello! It's always great to see more people learning about Ethereum.
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. Each instruction sent to the Ethereum Virtual Machine to process a transaction or smart contract costs a specific amount of Gas. If the required amount of Gas is not provided to the transaction, it will fail before completion.
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.
When you request that a miner add your transaction to the blockchain, the miner has to add it to a pool of pending transactions. The miner selects the transactions that have the highest reward for processing first, so the transactions that have high "bids" on gas prices will be most likely to be added first. The same goes for smart contracts.
So, in answer to your question, it's not so much the amount of Gas as the conversion rate you specify that dictates if your transactions are processed faster or slower. If you offer higher rates than the network average, you'll likely get processed faster, and vice versa.
Source:
https://www.reddit.com/r/ethereum/comments/6drxrd/what_is_gas_or_gas_limit_more_in_post_below/Other readings:
https://ethereum.stackexchange.com/questions/3/what-is-meant-by-the-term-gas
