I'm confused. Are you also a dual citizen (which Germany generally doesn't allow, but permits in certain cases) or are you just a German citizen with Global Entry [who is eligible for GE once cleared by the German Federal Police + application made & approved by US]?
If you have a Global Entry card, then that is a travel document in the sense that you can use it to enter the US from the Canadian border via land or sea. GE card is not accepted by air and a passport must be used at US CBP Preclearance locations. And a GE card is not valid for entry by air into Canada (although it is valid as proof of identity under WHTI to enter Canada via land/sea, you just don't get the NEXUS lane/benefits).
A Global Entry card can and should get you into CATSA Trusted Traveler Lanes. Despite having the NEXUS logo at the lanes, it is published on the CATSA website (and their employees know) that Global Entry members are allowed in the line.
US CBP lists multiple countries where their citizens can apply for GE (Aregentina, Australia, Canada, Colombia, Germany, India, Mexico, New Zealand, Panama, South Korea, Singapore, Switzerland, UK). NEXUS says US/Canadian Citizens, US Canadian Permanent Residents, and Mexican nationals in Viajero Confiable. No mention of German citizens being eligible for NEXUS at all.
