the notes in g chord are gbdgbg. why is the open g tuning dgddbd different?
The tuning for open G is actually: D G D G B D
The reason that it differs from the Open G chord as played on the guitar is that when you are using an open tuning, each string needs to be tuned DOWN to the note in the chord that is closest to the pitch of that particular string.
For example, D is closest to the 6th and 1st string E's, so they are tuned down to D. G is closest to the 5th string A. so that string is tuned down to G, etc.
You can find more information about open tunings at: