The gangster grip is only good for spraying bullets as the recoil moves the point of impact from side to side.
There is that, but it is also recognized by some gun experts to be reasonably accurate while you are shooting quickly.
I do not believe it started in the movies. It was popularized by movies, but I don’t believe it originated in movies or gangs.
Davis says it can be effective in the right situation.
The important thing is that you’re close enough to the target and that you have enough experience to hit it without having perfect aligned and centered sights. This takes a great amount of time in training and use with the weapon before you will be any good with flash sight picture.