When selecting that filter the dialog pops open.
As default “Output Mode” there is “Replace Layer…”.
Section “Output Folder” and “Output Filename” is grayed.
Now change “Output Mode” to “Save CLUT as .cube or…”
Bug:
The Output section stays greyed.
Activate “Preview” (“Vorschau” in my “German version”).
Output section gets selectable.
G’MIC v.3.3.2 PlugIn @ GIMP 2.10.32 @ Win7-64