It sounds like you want the surface area of a sphere.
https://www.google.com/search?q=surface+area+of+a+sphere&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari
Gravity is a little more complicated, but this page can help.
https://www.omnicalculator.com/physics/gravitational-force
Good stuff here. I love Sci-Fi. Please post when you finish your book.