While My Guitar Gently Weeps
by The Beatles
is most likely in the Key of C Major
Notes in C Major:
A, B, C, D, E, F, G
Chords in C Major:
C, Dm, Em, F, G, Am, Bdim
This key can also be known as A 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.