I can't comment on what was afoot before Vat.II with guitar music at Mass, but immediately after the council, there were local groups getting together to make the liturgy "more meaningful."
So I know this was happening in 1968, and from what I recall, it seemed to have some kind of history already by then. But I can't be definitive.
I know that Evlis Presley had his start in "gospel" music, mostly voice. And there was a large popular movement toward young people singing in small groups with one or more guitars present. Therefore, when they sought to make the liturgy "more meaningful," the first step was to import a guitar to the Mass.
There was a profusion of typewritten song sheets with guitar cords drawn in between the lines. Writing in the guitar cords was easier than having music staves that show two or more vocal parts. So some of this was due to expediency.