California has a law, dating from before Newsom was Governor, that prohibits State funds from being used to travel to certain States that have laws limiting LGBT rights (for example limiting female athletic competition to biological females). The Governor's office in coordination with the State Attorney General's office decide which States these are. Montana was on that list as of June 2021. There are 19 other States on the list as of this post, with two more States expected to join that list in the next month or so.
Newsom says that the law does not interfere with private travel, only State funded travel. Assuming that Newsom paid for his own and his family's travel that would not be out of compliance with the law. However, Newsom's security detail accompanied the family to Montana and their travel cost is funded by the State.
Newsom has apparently, never said, "No Californian should travel to Montana" or any similar statement so despite some people calling him a hypocrite, I don't think this makes him one. He may however, have broken California law depending on how a court would look at it and he may have to reimburse California for the expense of the security detail.
An article on this situation is here.
Attorney General's statement in 2021 on this is here.
One interesting thought is that since UCLA and USC are both joining the big ten (which currently has 16 teams), there will be (because Iowa is on the list) more State funded travel to States on the 'banned' list.
