G, C, D is a I, IV, V progression, the most common chord progression used in rock music. (I) refers to the root chord G so a scale of G major will hit all the notes in that key. (IV) is the sub dominant chord and (V) is the dominant which leads back to the root (I)

If you want to use the Em scale over the progression that will work because Em is the minor sixth of the key of G. It utilizes the same notes as a G Major scale.