|MadSci Network: Physics|
Energy can be in many forms; in this case the kinetic energy (energy of motion) of whatever is escaping is converted into gravitational potential energy. "Heat" energy is due to thermal motions of atoms within an object (or gas, or liquid) and as such is often also called thermal energy. While some of the energy from a rocket is converted into heat, that doesn't actually help the rocket escape from being gravitationally bound to a planet (or any other body). Since you have to escape gravity, you need to convert your energy into gravitational potential energy.
Of course, in this system you also have chemical energy - it's what is fueling the rocket! So you convert chemical potential energy into kinetic energy (as well as some heat, as I said before). Then that kinetic energy is converted into gravitational potential energy!
To answer your other question, gravity does not "pull" on a field - gravity is a field. It is similar to an electromagnetic field, except of course it is not charges that cause it, but rather mass.
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