Thank you! Somehow BRouter still ignores that gate when the "Randonneur" profile is used, but I think that's a bug in the profile:

https://bikerouter.de/#map=15/48.7748/5.8182/osm-mapnik-german_style,Waymarked_Trails-Cycling&lonlats=5.826101,48.769258;5.816189,48.776489&profile=randonneur