The disable option in the GlobalProtect client is greyed out because the client cannot be disabled.
The disable option will be greyed out/not available if on-demand option is checked in the portal configuration in the firewall. In on-demand mode, the user has the ability to connect and disconnect whenever required. So the client is automatically disabled by clicking on disconnect.
Make sure to check the option single sign-on to use the option "disable". Refer to the screenshot below.