To start at the end - yes. God is always doing something about
gravity. I think what you mean is "Can God do something differenr from
what he normally does via gravity?" Yes - but God does so very seldom.
In the framework of classical physics, in which one pictures
everything happening continuously ("nature does not make jumps"), we
would also picture God acting continuously with fields & particles. In
quantum theory nature does "make jumps" whose precise results are not
predictable. IMO God acts to determine things as much as the laws of
nature (approximated by our equations of QM) allow - but not more so.
On Booth & the bullet - the analogy is limited because it might
invite the idea that God starts off each occasion but then lets it
develop on its own. Pannenberg has argued that the idea of inertia was
a major factor leading people to think that God didn't act in the world.
(You don't, as in Aristotelian physics, need a force to keep things
moving.) This is perhaps a place where the energy concept is helpful,
both in physics & theology. God's "operation" (Greek _energeia_)
co-operates (literally) with the energy of physical systems to make
things happen. & a free particle needs to have energy to keep going.