Hope this will be helpful to someone:
Today received a KIG300 to control two shutters. I could install the gateway in the Velux Active App and control the shutters with the Velux app. I could also control the shutters with the Apple Home app on my Iphone.
But I could not add the shutters to HA, I got the error message “No unpaired devices could be found”
Then I deleted the Gateway from the Apple Home app and then the “VELUX Gateway” suddenly appeared in the HA Homekit Controller to configure. After configuration the shutters can be controlled from HA.
I found the tip to delete gateway form Apple Home app here: Home Kit Devices Not showing in Home Assistant ,
That’s the issue: HomeKit devices cannot be paired to more than one controller (hence the “no unpaired devices could be found” error). So you have to remove them from the Home app on iOS, possibly reset them (device-dependent), then add them to HA.