The electronic configuration of polonium is 2,8,18,32,18,6. It shall gain two electrons. How can it be a metal?
Polonium is a metal because of its physical properties. This is true, even if its number of valence electrons seem to make us think it should be non-metallic.
With polonium (and several other elements of high atomic number) we have an element with n=6 as the valence shell. For elements like these, trying to use a simple rule such as "elements having 6 valence electrons must be non-metals" clearly is not working.
You would be better advised to classify metals as elements having low ionization energy and low electronegativity rather than trying to judge strictly on the based of number of valence electrons.
This helps us understand why the boundary between metals and non-metals is not a vertical line on the periodic table, but rather angles down and to the right. (Remember that ionization energy decreases down any column of the table.)
With polonium, its valence electrons are so close to other empty orbitals, that metallic bonding can occur. Low ionization energy makes it possible for the electrons to move from one atom to another as the type of mobile electron sea that we find with metals.