Two things you can get cracking on right away are:
- Check out the developer documentation
- Browse the source code
You’re looking for information on how the developers have coded the user permission check when they attempt to create a form. It might be a current_user_can() + capability combo, the use of custom filters & actions, or a combination of the two.
Once you know this, you can “hook” in with your own plugin & extend the checking process. This will involve determining how many forms the current user has already created, so again you’ll need to refer back to the above, plus most likely an inspection of Gravity’s database schema.
I don’t think the plugin you’ve mentioned will come in handy since it deals with posts, whilst Gravity is something quite entirely different.