The atoms combine to form compounds to attain stability in nature. The combination of atoms takes place by sharing of electrons between the atoms or complete transfer of electrons from one atom to another. Generally, atoms combine to complete their octet, that is to possess eight electrons in their outer most shell (noble gas configurations) except hydrogen which can attain stability by two electrons in its outer most shell.
Since germanium has 4 electrons in its outer most shell so it needs 4 more electrons to complete its octet and attains the stability. Hydrogen has 1 electron in its outer most shell and it needs only 1 electron to attain stability so, each germanium will combine with 4 hydrogen atoms and thus forming molecule which is stable in nature.
Hence, is the formula of the hydride formed by germanium.