We do not really know. It is actually a very difficult experiment to conduct because at the scale of the atom, and sub-atomic particles, gravity is so weak it hardly affects anything. Near the Earth, bodies fall at 32 feet/sec/sec or about 1000 centimeters/sec/sec vertically. Anti-protons and anti-electrons when they are produced in 'atom smashers' usually live only 100 millionth's of a second. During this time, gravity would cause them to 'fall' or 'rise' by only 1/2 x 1000 x (10^-8)^2 = 5 x 10^-14 centimeters. This is about the diameter of an atomic nucleus! It is an important experiment, however, and physicists are working on this test even now.