According to Newtonianism (and maybe other approaches as well), all objects are supposed to fall at the same rate in the same gravity regardless of mass. Yet, as particles get lighter, they become more likely to escape gravity wells with lower amounts of energy/momentum. So a water molecule can evaporate off the moon, whereas Earth's gravity is too strong for that to happen to water, although it could happen to helium on Earth.

 

Helium floats not because it interacts different with gravity, but because its density is much lighter than that of other gases that comprises the atmosphere. It is just like oil floating on water.

Hear's my take on it.

 

Light will always travel by the shortest route between two points. This is called the geodesic. In a curved space-time, the geodesic is a curve. Curving space-time is gravity. So in the presence of a gravitational field, light follows the curve of space imparted by that field. ( or the whole thing is a matter of the geometry of space-time, either way the result is the same.)