Passport.js authenticate to Gmail API leads to GooglePlusAPIError - oauth-2.0

Question

I use Passport.js to authenticate with Google. I define the following scope:
passport.authenticate('google', {
scope: [
'https://www.googleapis.com/auth/gmail.modify',
],
})
So, I just would like to access the Gmail API.
Now, Google responds with:
GooglePlusAPIError: Insufficient Permission
I get it working when I add the Google+ API to my projekt via the Google API dashboard and by adding the following scope to my passport.authenticate call:
'https://www.googleapis.com/auth/userinfo.email'
But why do I need the Google+ API when the only thing I would like to do is access Gmail?
I also found this website: https://developers.google.com/gmail/api/auth/web-server where they set these scopes:
SCOPES = [
'https://www.googleapis.com/auth/gmail.readonly',
'https://www.googleapis.com/auth/userinfo.email',
'https://www.googleapis.com/auth/userinfo.profile',
# Add other requested scopes.
]
What am I missing?
Thanks in advance!
Cheers,
Niels
Edit:
I just researched on my Google API dashboard and found the screen below. So, are these scopes mandatory?
Edit 2
The context is an Express.js app. The whole call looks like:
router.get('/google', passport.authenticate('google', {
scope: [
'https://www.googleapis.com/auth/gmail.modify',
],
}));
router.get('/google/callback', passport.authenticate('google', {
failureRedirect: '/',
}), (req, res) => {
req.session.token = req.user.token;
res.redirect('/');
});