It’s true that passing a law today which bans guns will not prevent any incident tomorrow, but common sense says that regulating guns and lowering the number of guns around, in the long run, will reduce crimes committed with guns.
Aside from common sense, just look at the level of gun violence in countries similar to ours, but with tighter regulations (and enforcement). It’s lower.
I really hope your grandmother doesn’t have a gun, because statistically, someone owning a gun is more likely to have some sort of incident useing that gun against themselves or a loved one than have an incident in which they’re protected by that gun.