GTK LAF does not gray out disabled JMenu

FULL PRODUCT VERSION :
openjdk version "14-ea" 2020-03-17
OpenJDK Runtime Environment (build 14-ea+22-982)
OpenJDK 64-Bit Server VM (build 14-ea+22-982, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
RHEL7, RHEL8, SLES12

A DESCRIPTION OF THE PROBLEM :
A JMenu with setEnabled(false) is not grayed out with GTK Look and Feel.
In SwingSet2 demo, "Themes" menu will be disabled with GTK, Nimbus, or Motif LAF.
However, GTK LAF doesn't change the color of the menu.

This problem didn't occur with GTK2, by "-Djdk.gtk.version=2" option.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. java -jar SwingSet2.jar
2. Switch Look and Feel to GTK

EXPECTED VERSUS ACTUAL BEHAVIOR :
Expected:
"Themes" menu item was grayed out.
Actual:
"Themes" menu item was still displayed without change.

REPRODUCIBILITY :
This bug can be reproduced always.