Put antifreeze in black and grey after winterizing ?

I am not equipped with by-pass, but if I were I would leave it open to allow for expansion.
One half gallon is more then I would be putting in the drains. A quart would be sufficient.
Yes, some anti-freeze in the grey and black to protect the valves as "mountainkowboy" suggested.

A good flush in the spring along with sanitizing is a good idea. Use backing sode to kill unwanted tastes.