In many cases GnuGk can act as a drop-in replacement for the ECS Gatekeeper. I just noticed one strange thing: Radvision MCUs seem to register endpoint aliases instead of prefixes with the ECS and the ECS treats registrations from MCUs as prefixes. To fix that, you can simply assign prefixes to the MCU in your GNU Gatekeeper configuration and everything works like before.
Voila!
