A 256-bit hash which proves, combined with the nonce, that a sufficient amount of computation has been carried out on this block: the Proof-of-Work (PoF).
An optional free, but max. 32 byte long space to conserve smart things for ethernity on the Blockchain.
The gas limit is maximum amount of units of gas you are willing to spend on a transaction. This will decide by community based on your opcode set.
For example if you have a contract that will add two numbers, gasLimit maximum amount of units to execute function. This list will find below link