RSK has a “minimumGasPrice” at block level, this value is set by the miners and can be negotiated by them with a variation of 1% between blocks. A transaction with gasPrice lower than minimum is rejected.
Actually, Nifty takes this value as gasPrice for the transaction. This is ok in a “steady context”, but when the value is under negotiation, this approach could be a problem (Nifty takes MGP and -before broadcasting- a new block with a 1% higher MGP appears that makes the transaction invalid).
What is recommended here is to set minimumGasPrice + 10%, because this gives a sufficient margin to the tx to be valid and mined.
Please let me know if you need further explanation or have any doubts.