Total GCR dose is 3-5x in transit to Mars compared to what you get on ISS; on the Martian surface it's from 1.5-2x the ISS dose. (see https://www.swsc-journal.org/articles/swsc/pdf/2020/01/swsc2...).

On a long-stay Mars mission, that adds up to 12-18 times the accumulated GCR exposure compared to a six-month ISS increment.