A G5 (G Power Chord) chord contains the notes G, D. It is built from the formula 1 - 5, with G as the root.
Should I play power chords with two fingers or three?
Three is the standard: root, fifth, and the octave of the root on top. The two-note version (dropping the octave) is thinner but perfectly valid, and it's easier to move fast. Most players learn three and drop the top note when the tempo demands it.