Question

Why did Mendeleev leave some gaps in his periodic table? Besides gallium which other element have since been discovered that were left by Mendeleev in his periodic table

Answer 1

Mendeleev leave some gaps as he was assure that there were elements that were not discovered at that time and when they will be discovered they can be easily fitted into table. scandium and germanium were discovered afterward

Answer 2

Mendeleev left some gaps in his periodic table because he believed that in future more elements will be discovered. He predicted the existence of some elements and named them by prefixing a Sanskrit numeral, Eka (one).
Scandium --- Eka-boron
Gallium -- Eka - aluminum
Germanium --- Eka-silicon