#1
On the song "Knockin' on Heaven's Door", what scales would you use to solo with? -Using G-D-Am G-D-C chord progressions in standard tuning? I've heard that E-Pentatonic works well. Thanks!
#2
You have 3 options:

1. G major scale
2. G major pentatonic
3. G minor pentatonic

Here's why:

You've heard that E minor pentatonic works well, but that's because its the same notes as G major.

The rule of thumb is you can play the major scale and associated pentatonic of whatever key you are in.

Major key= major scale.

Minor key= minor scale.

You can also use the minor pentatonic in a major key for that "blues" sound. The reverse is not true.

Work on that and it'll get you pretty far. You wont need to learn more scales for a while.
"There are two styles of music. Good music and bad music." -Duke Ellington

"If you really think about it, the guitar is a pointless instrument." - Robert Fripp