Gingsberg stole it from Yeats — “the best lack all conviction…” / “the best minds of my generation…” — many similar verses, e.g., “what rough beast…” / “what sphinx of cement…”
https://www.poetryfoundation.org/poems/43290/the-second-comi...
https://www.poetryfoundation.org/poems/43290/the-second-comi...
The best lack all conviction, while the worst // Are full of passionate intensity.
vs
I saw the best minds of my generation destroyed by madness, starving hysterical naked, // dragging themselves through the negro streets at dawn looking for an angry fix,
How is this stealing in any form?