I have verified that I can use the service account credentials to impersonate a user for Google Sheets as shown in this tutorial. Note though that the credentials user interface still gives an error message (but they work!) which is described in this github issue (2397).
However my original problem persists, since I want to use Gmail API, and here I still get an error even if I just “ignore” the error in the credentials UI.
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Am I lacking any scopes for gmail?
What scopes are needed?
**Edit: **
I found this this definition of scopes regarding gmail.
After I added all those, it started working!
However - Necessary scopes for these nodes should really be available in the node documentation!