I don't have any idea about the Gravity form. Is there a setting while registering the users to set their role? If yes, then you should be able to do it by setting the default user role to “Seller”.
In addition to getting the form to automatically active the seller, I've run into an issue with trying to set the user_nicename for the store url. user_nicename isn't stored in user_meta, but in wp_user.
I have actually unfortunately managed to find a solution for this yet. If the seller permissions were actually WordPress capabilities instead of user_meta/wp_user then I could use the User Role Editor plugin to auto-assign capabilities for new sellers registered to my website without using the Woocommerce form that Dokan uses.