Written by
Chris Columbus (Future director of "Home Alone" and the first two "Harry Potter" films) as a writing sample, the original version was much more violent and featured scenes such as the Gremlins decapitating Billy's mom and rolling her head down the stairs when Billy got home. They also ate the dog!
Another major difference is that the Mogwai Billy gets originally became evil and became Stripe, the script eventually ended up in the hands of Steven Spielberg who asked Columbus to tone it down, thus turning it into the movie we know today...