Writer Gerry Duggan announced the cancellation of Guardians Of The Galaxy with issue #150 because the story he pitched to Marvel was too big for just a Guardians title.
Folks, Guardians Of The Galaxy IS wrapping at #150, but it's because I pitched a story to @Marvel that was way bigger than just Guardians. HUGE stories are coming your way beginning in February. More soon. pic.twitter.com/f4OQX1vGMf— Gerry Duggan (@GerryDuggan) December 5, 2017
Issue #150 sees the return of Adam Warlock and the book's events will continue in February in Infinity Countdown: Adam Warlock and Infinity Countdown Prime. Meanwhile, Guardians Of The Galaxy will eventually be relaunched with a brand new #1 and a half dozen variant covers.