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I wonder if it will use the same GCKEY for my sponsorship as for my Canadian citizenship.

you cna link to a new or existing GCKEY acc. If u already have one, don't make a new one, once u get ur AOR code, link it to that.
 
you cna link to a new or existing GCKEY acc. If u already have one, don't make a new one, once u get ur AOR code, link it to that.

So, there will be instruction on how to link it? Do you think my UCI will stay the same as it did for my PR card and citizenship app?
 
If I'm both the sponsor and representative of the principal applicant, can I link the application before AOR (using name and birth date) or do I have to wait till AOR?
 
If I'm both the sponsor and representative of the principal applicant, can I link the application before AOR (using name and birth date) or do I have to wait till AOR?

You can try but low likelihood. Generally linking possible within a couple days of AOR (before that is) or similar like biometrics. Ie basically same time as aor