Hello,
No it does not effect the barcode as IMM5669 does not produce barcodes; it just validates. GC key is your account once you are successful in linking your paper application to this online account by answering questions linked to your paper application. The link to making a GC account is here
http://www.cic.gc.ca/english/my_application/link.asp
You can also find information on what this is, on the CIC website.
Once you are successful in linking your account, you will see that they have requested the submission of this form through your account. You can see this by going to "view submitted application", once your application has been linked.
Once linked, if you do not see this request within a week of linking your account, or if all else fails, and you are still confused, just mail your IMM5669 with a letter explaining that you were unable to link your account therefore, sending it through mail. Make sure, if you are mailing your form, the PA signs the form with hand-written signature.
Best of luck!