If you look at the Parity chain spec for mainnet (https://github.com/paritytech/parity/blob/master/ethcore/res/ethereum/foundation.json), you will see this line:
But that should mean that the gas limit is 5000? And that's really strange!
Someone is going to say "That is only for the genesis block" and then I will ask them to please answer this question:
Someone else may say "you can change it to any value you want" but that is not the point.
This is also related: