From the Super Mario Wiki
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He is also one of the few Cragnons to not get brainwashed and turned into a slave of King Croacus IV. While Gabbro was captured and brought to the Floro Caverns, with the help of Dottie the Pixl, he was able to hide from the Floro Sapiens. When Mario, Peach and Bowser arrived, Gabbro thought they were Floro Sapiens until Dottie corrected him.
Dottie wanted to join Mario's team, but Gabbro did not want to lose his friend who helped him escape. Dottie replied she could only be with the people who knew how to control the powers of Pixls. With that, Dottie shrank Gabbro so he could escape the Floro Caverns without getting seen. Gabbro went through a small door and presumably returned to Downtown of Crag, though he is not seen again.