Ummagumma has Gilmour's first solo writing credit.

Yep. It’s “The Narrow Way” on the studio half of Ummagumma (1969). Each band member contributed solo material, and Gilmour’s three-part suite was his first writing credit without Waters or Wright.

Funny enough, the song isn’t that memorable—even Gilmour’s called it “unfinished.” But it’s a milestone. He’d get better.