Here's what worked for me (edited from OuIrish's advice):
1. Log in with GCKey account
2. Permanent Residence - Family Class (Spouses)
3. Application Number and Unique Client Identifier (UCI) - Insert application number and UCI from email
LEFT THIS BLANK: 4. City of Birth - Insert PA's City of Birth
LEFT THIS BLANK: 5. Country of Birth - Select PA's Country of birth
6. Passport Info - Insert Passport number, issuance and expiration info
7. Other Application Info - Fill out the other application info as per your application (Province, City and marital status) and put '2' as the number of
members in the application for sponsor + spouse (add +1 per dependent, if any)
8. Click on the 'Search for my application' tab
9. Then on 'Link my application' tab