is most likely in the Key of G Major
Notes in G Major:
A, B, C, D, E, F#/Gb, G
Chords in G Major:
G, Am, Bm, C, D, Em F#dim
This key can also be known as E minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.