@Yalba @David_Tschumperle I tried using GIMP-2.9.8 18794a6ba2 and G’MIC-Qt-2.1.5 and it works fine here.

Tried using both default and dark themes, and with “Use native color dialog” enabled and disabled, with GIMP restarts in-between - always worked fine.
I self-compiled G’MIC using Qt-5.9.3 apparently.
@Yalba have you tried changing “Use native color dialog”?