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
Gloam Valley becomes accessible after finding the orange Heart Pillar in the Flipside Outskirts. The dimension introduces pools of water meant to impede the player's progress, with Princess Peach's parasol often being the only way to cross them and proceed. Later on in the chapter, Boomer is found and joins Mario's team, and his explosive abilities are used to continue forward and reach Merlee's Mansion, the setting for the remainder of Chapter 2 and the resting place of the third Pure Heart.
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