Gravity must decrease due to less effective mass when going inside the object but also must increase with depth inside the star due to its higher density. Is there a model or formula approximating gravity calculations along the radius (from center to surface) of the stars?
You need to know the equation of state for the star's interior. Once you know this you can calculate the density variation with depth and the gravity inside the star.
Google for something like "star equation of state" to find lots of articles on the subject, but note that it's exceedingly complicated because there are so many factors at work. This is the sort of article you'll find: good luck reading it!
Note also that while we can use models to calculate equations of state, the results are only as good as the models. It's hard to know how good our models are when all we can see is the surface of the star.