Thank you. But it seems that it is the command to prevent auto-update, not to install older version.
I found the solution. First, with “flatpak install org.gimp.GIMP” install GIMP latest version, and next, downgrade GIMP to ver. 2.10.38 with the command below.
sudo flatpak update --commit=dc9f60dce7f8ee883fc6a771fb4eb4678b732b1c6d9f0559998a226b114ad4f4 org.gimp.GIMP