1) GPE is a negative quantity when it is defined that GPE = 0 at a distance of infinity from the centre body.
It is known that gravity weakens the further you are from the centre body.
Gravity is an attracting force, meaning as you move in the same direction of the field lines, you lose gravitational potential energy (similar as to how positive ions moving in the same direction as magnetic field lines lose electrical potential energy). As it is 0 at infinity and GPE decreases the closer you are to the centre body, GPE is negative within the gravitational field.
2) GPE is defined as the energy required to move an object from a point of infinite distance from a centre body to a point within its gravitational field.
This means it's GPE varies according to its distance from the centre body.
When an object is moved towards the centre body, as it is moving in the direction of the gravitational field lines, it loses gravitational potential energy, which is converted to kinetic energy as it moves and so negative work is done (loss in GPE).
When an object is moved away from the centre the body, as it is moving against the gravitational field lines, it gains gravitational potential energy from the kinetic energy required to move it to that point and so positive work is done (increase in GPE).
You can gather how to answer that last question of yours from what i've already said.
If you're still unsure, feel free to ask questions.
