ok. the gravity gun needs to be a weapon with the fastes possible fire loop and infinite ammo.
entities that can be "gravitated" need to be given a custom script.
There is no chance that you can "lock" the entity to the crosshair, as FPSC doesn't support this, although you could use the native commands "moveup" "movefore" and "moveback" to control the crate.
Heres how it works.
The entity has infinite health (strength=0). The weapon has infinite ammo and fires very fast.
the entity script is something like this:
:state=0,shotdamage=1,"G"KEYPRESSED:movefore=1
:state=0,shotdamage=1,"H"KEYPRESSED:moveup=2
:state=0,shotdamage=1,"J"KEYPRESSED:moveback=3
The tect in capitals needs to be changed to the correct command for a key input.
basically if you press the key while the object is being damaged it'll move.
God knows if this will work, I just thought of it in my lunch break.