I'd say as many as 7 were swung on the gun vote, and there are around 17 real swing states(Rest at least lean one party or the other). Even if it was 4, that's almost 25% of the swing states.
If I lost 50,000 voters in Ohio or Michigan, and gain 100,000 in California, I'd rather take the Michigan or Ohio voters, since those elections frequently are within 5%.