You can also modify the .py script. One line 194 & 195, insert you sub-folder:
Code:
Line 194: candidate_dirs = [ os.path.join(os.path.expanduser("~"), "AppData", "Roaming", "GIMP", "2.10", "plug-ins", "Johnak_GIMP_Color_Grading"),
Line 195: os.path.join(os.path.expanduser("~"), ".config", "GIMP", "2.10", "plug-ins", "Johnak_GIMP_Color_Grading"),