Hi Murry, are you hanging domestic guitars from the neck until dead?
Hope you're playing them before killing 'em. Hope that idea wen well.

I believe you're looking for toggle bolts/screws or hollow wall anchors. The toggle is stonger IMO but that also depends on the thickness of the sheetrock (5/8") at least if it's more than a hanging guitar, unless you place some sort of backing plate behind or in front of the sheetrock or a stringer, say a vertical length of wide strapping that you'll then attach the shelving to, that'll take & spread the pressure of the area of the toggle. You are always best going into the wall studs but I'm sure you already know that.
Good luck, you ever coming back over for a sing & a visit, don't make me come over there.

All my best
Barry