@sdetweil a friend of mine found the solution; I had to fill in my gmail-adress as a “test-user” (I haven’t seen this in the tutorials I watched) , then the token was created and now the module is working fine!

Hey everybody,
over the past days I tried to configurate my own smart mirror. My knowlegde about computer programs is really very very limited, therefore I was quite happy that I was able to install a few modules.
Right now I try to install the GooglePhotos module, but whenever I try to generate a token and log in to my Google Account I receive the following message: " The developer hasn’t given you access to this app. It’s currently being tested and it hasn’t been verified by Google. If you think you should have access, contact the developer."
Has anyone of you ever had the same problem and can help me?