What is the definition of "gasLimit" in the chain spec files for Parity? The wiki says it is "gas limit of the genesis" ! If I knew what that definition meant, I wouldn't need a definition.
Each ethereum transaction uses "gas" to protect miners from running infinite computations. Transaction senders may send a gas limit which indicates how much gas they are willing to spend on processing a particular transaction (in general it is impossible to know in advance how much gas a transaction will require).
A block also has an overall gas limit, to prevent blocks from getting too big, and processing times from getting too high. The genesis block is no different in this regard - the setting you refer to is the gas limit for the genesis block, i.e. how much gas may be used when mining all transactions in the genesis block.