You don't win anything in another state if California passes the bill. You may think you don't lose anything either, but it sets precedent that other politicians around the country take note of. It normalizes further erosion of freedoms and liberty nationally.
California is the most populated state in the entire nation. Once something is done in California it is much easier for it to start showing up in other places.
California was the primary source of the national level Assault Weapon Ban for example. Something that would have become just as permanent as the NFA or GCA if it did not have a sunset provision. If that sunset provision had come in 2009, or even 2006 instead of 2004, it would have just been signed back into permanent law.
Fortunately it expired, now it has been dead long enough that it would face stiff resistance even by some Democrats. But in 2004 the only reason it was not permanently renewed was because the Republicans controlled everything. Even then most were completely unwilling to oppose it, they just let it lapse without standing for anything.
If it had been renewed, it was going to become permanent, and as likely to be removed in the future as the GCA or NFA are today.
California has also been a major melting pot of people from across the nation for decades. That is changing now, as more begin to leave than come to California, but it means California has been a good indicator of the nation as a whole. The extreme antis of the East, and the extreme pro-gun of the West and South (other than the Jim Crow type stuff.) You had country places that would remind you of somewhere in the South in California for decades. Even some big metro areas like Bakersfield were more "Country" and southern in culture than some areas of Texas.
California is not all left leaning liberals. However much like the Eastern United States from Chicago to Boston down to D.C. (and primarily anti) has the greatest population in the United States, the San Francisco/Oakland and Los Angeles areas (primarily anti) dominate California in population.
So the sum of California has long been similar to the sum of the entire US. The most anti places in the US have the greatest density of people (From Chicago Metro area, to Boston, down to NYC, and into D.C.). The most pro gun often have the lowest concentration of people (Alaska, Montana, etc) There is exceptions, but those are the trends.
California has both, and the ideologies of both. It also has the cultures of most other portions of the nation in concentrations not that different from the nation as a whole.
So anything that can happen in California can certainly happen nationally.
California is around 12% of the nation's population and 13% of the GDP of the entire nation. It has many of the elements of the other 88%. More so than any other state.
You do not want to see California laws get worse.
Here is a population density map, notice the majority of the large concentrations of dark blue areas are in more anti stronghold areas. Florida being a major exception. (North Carolina as well, though they have some Jim Crowe things still in place, like a pistol purchase license requirement.)