If you plan to uninstall the Webgate that is configured to operate with the ISA Server, you must first unregister the Access Manager filters manually before you uninstall Webgate.
Registering and Managing 10g WebGates with Access Manager 11g for details about uninstalling 10g Webgates
To unregister filters before Webgate:
regsvr32 /u ISA_install_dir\access\oblix\apps\webgate\bin\webgate.dll
regsvr32 /u ISA_install_dir\access\oblix\apps\webgate\bin\postgate.dll