Why do you call them Grandpa Sun instead of Grandma Sun?

This is because it is called "the sun".

China's theory of Yin and Yang holds that light is Yang and no light is Yin. The sun is the pole of yang, so it is called "the sun". In the correspondence between Yin and Yang, Yin is female and Yang is male. Therefore, it is called "Grandfather".

Correspondingly, the moon was also called "lunar" in ancient times.