Hi Ann,
If your School gives you a school issued email address they may wish for you to use that over your other email addresses. I seem to recall that once the accounts are linked, only Canvas Support (available via the help menu) can unlink them to re-sign you in.
This very well may have happened the first time when you were already signed in using your GCTS email account on Google and when you pressed the Authorize button, it linked to that account.
I would first recommend checking with your school's IT support at https://gordonconwell.edu/technology-services/ on what their policies are for linking to a non GCTS account and then if they are OK with it, contact Canvas Support to ask to break the link so you can re-link to a different Google account.
I hope that helps, please let me know if you have any further questions and I know the community will be happy to help out!
Stuart
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