A Gsus2 chord contains the notes G, A, D. It is built from the suspended 2nd formula 1 - 2 - 5, with G as the root.
What note changes between G and Gsus2?
G major's third is B; Gsus2 replaces it with A, the second. On the open G shape that means letting the B string ring open (A is already covered elsewhere in the voicing) rather than fretting the third — leaving a fuller, more open-sounding G.