To keep the analogy to gravity, and I think to more accurately calculate the effect of members on the overall community, the amount of gravity each member contributes to the overall gravity should be inversely proportional to their distance.

Or, to put it in community terms, if two members have identical reach and love scores, but different orbit levels, the member in the closer orbit (say, Advocate) has a larger gravitational impact than the member in a farther orbit (say, Observer). This is because members in the inner orbit are more likely to interact with any given new member than those in the outer orbits.

I propose changing the definition of Gravity to be:

Gravity = Sum(Love * Reach / Orbit^2) for each member

This also lets you calculate the relative gravity of each orbit in your community. If you have more gravity in your outer orbit than your inner orbit, that means members in the middle are feeling more "pull" towards being less-involved than being more-involved.