Question

Why Gravity Is not repulsive ?? What are anti-electrons??

Answer 1

There is only one kind of mass, (unlike charge, which is two type). So the force can be only one type. Now why this one type of force is attractive is not well understood yet.

An anti electron is a particle which has mass equal to the electron. Charge and other properties are exactly opposite to the electron. It is called positron. If one anti electron collide with electron they will annihilate and produce photon.