[Ethereum] Setting gasPrice not working on private network

how is this default value generated

This is a median based on the value used in recent blocks.

will it changes now and then according to the market

Yes, it changes.

transaction creators hope the lower gasPrice while the miners opposite，how to reach an agreement miners could start the geth with the parameter --ask to set the lowest gasPrice，will the setting changes during the runtime?

It's a market mechanism.

Miners set the minimum price at which they are prepared to accept transactions. This can be either set on the command line when the node is started or changed while the node runs. Transaction creators set the maximum price which they are prepared to pay for their transaction to be mined.

Miners want to maximize their revenue. There's a cost to accepting a transaction, because a bigger block will take longer to propagate across the network and get validated by other miners, resulting in a greater risk that the block will be orphaned. There's also a maximum limit to the gas that can be used in the block, beyond which it is invalid, so if they fill that up they want to fill it at a high price, not a low price. However, if they set their price too high then they'll lose money be turning away too many paying transactions.

You can not change the gas price of EVM instructions without hacking the core and totally forking off to a new client. This is true for both Geth and Parity.

If the gas limits are not sufficient for you, you can just increase the following configuration options for Parity:

--gas-floor-target GAS         Amount of gas per block to target when sealing a new
                               block (default: 4700000).
--gas-cap GAS                  A cap on how large we will raise the gas limit per
                               block due to transaction volume (default: 6283184).

You could try to start with a floor of around 9999999 and a cap of 99999999. And see if this works out for you. Now we covered the limits.

If you want to override gas prices to be accepted by miners, you have to change the following parameter:

--gasprice WEI                 Minimum amount of Wei per GAS to be paid for a
                               transaction to be accepted for mining. Overrides
                               --basic-tx-usd.

You could try to set it as low as 1 Wei.

And to cover your last question: You can not specify gas price in a genesis block.