I agree with Iwrite that no writer gets a cut of the gross, especially the new writer.
The following is my opinion based on years of observations. You don't have to agree or disagree.
However there have been exceptions and ways around the system in place. It all depend on how badly they want your work. The more the desire the greater chance your agent can rake them over the coals vs. them you.
Simon Kinberg is a good current example of ways a writer can get more than the usual. Mr. and Mrs. Smith was his first script sold, even though XXX part 2 was made first, and was actually his thesis script at college. He sold it for mid against high sixes with no points but managed to get attached as a co-producer (in name only) so he could get a cut.
In many cases writers only get compensated an amount equal to 1 to 2 percent of the below the line budget with a floor and a ceiling. Such was the case with my last sale.
When you hear low against high six, the low is the floor and the high is... you know.
So as an example: Let's say the deal is 1% of the below the line budget with a 100K floor 500K ceiling . If greenlit at 50 million, the writer would get 500 thousand. If it ends up a 100 million dollar film to make, it still is 500K to the writer.
Plus it is fairly common to get an extra bonus if the budget hits a certain number above and beyond the ceiling. Less common is a distribution bonus whereas if the film makes a ton of money the writer gets a small bonus. Even less common is points on the film's gross.
As for Gump (and many other films) making a ton of money and the writer getting very little. Oh well. Get used to it, unless you become a producer.
The risks were taken by the producers. They put up the money and recourses to get the film made, not you.
A producer thinks: If you were to get more for a box office success, then so would the DP who made the film look so good, or all the actors for making the performances come alive, or even the grip, who made sure the electricity flowed well. Who do you think you are?
For the most part, producers regard writers as just another member of the crew. You got paid for your job.
Most Directors only get a cut of the net as well. Only the big, well established box office people, or people with clout-- whether actors, directors and/or writers, get a real percentage of a film.
But then again, it all boils down to how hot the script is, how good your agent is and who the buyers are. Trust that your agent will get the best deal for you.