Oh! sorry! GDG and GDGB. Very thanks!
Partial Answer: an X5 chord means you play the root note and its 5th, leave out the 3rd that makes it major/minor chord.
A5 is A+E, E5 is E+B, G5 is G+D. In Gdg tuning you just bar across the fingerboard and you are playing the root, the 5th, and the octave of the root. (or any 1-5-8 tuning)
To play an X5 chord with Gdgb (open G Major) you can either not play the high b string or fret it with your pinkie 3 frets higher than the bar you make with your first finger.
in 3 strings A power chord ist the same A5 ?
In four strings,I understand your answer very thanks!
Yes, the X5 chords are also called power chords, they only have 2 distinct notes, as allow as many multiples and octaves of the 2 notes that you want.
i.e. Gdgd' 4 string G5 tuning, G and its fifth D and then repeated an octave higher, still plays G+D
also Gdd'g'g' and Ggd' and G'Ddgg'd"