The discovery of new elements such as gallium demonstrated the predictivinnes ability of the periodic table proposed by Dimitri Medeleiev.
Mendeleieve observed that, when the elements were organized by ascending atomic mass, a periodic pattern of the properties appeared. The elements arranged in sane columns showed similar properties. So Mendeleiev predicted the existence of elements that had not been discovered but that he affirmed that should have some properties given the position that they would occupy.
Gallium is an example of one of the elements that was discovered and exhibited the properties predicted by the space that it occupies in the periodic table.