From the Super Mario Wiki
Gloam Valley is the setting of Chapter 2 in Super Paper Mario. The valley itself only appears in the first section of the chapter.
Chapter 2-1: Bogging to Merlee's
After finding the tower of the Orange Pure Heart in the Flipside Outskirts, Mario and his friends access Gloam Valley through an Orange Door. Mario treks through the valley and meets Boomer along the way, who joins him in his quest. Mario eventually reaches Merlee's Mansion and continues his search for the Pure Hearts.
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