There is no reason to wait for TN to expire, and every reason to keep TN valid throughout. Start the Gold Card process when the applicant has confirmed EB-1 or EB-2 eligibility, has the funds lawfully available and has taken tax advice, and renew TN on its own schedule.
Run the two clocks in parallel and let neither lapse
The Gold Card sequence and the TN cycle answer different questions and do not synchronize. TN admissions are granted for up to three years at a time and renewed on the strength of continued professional employment; the Gold Card leads toward an immigrant category whose timing depends on vetting, the gift and visa availability. Hypothetical example: the South Frontenac architect's employer is willing to file a TN renewal by I-129 six months before expiry; he should let that proceed regardless of where the Gold Card file stands, because a gap in status ends his ability to work.
Meanwhile, the Gold Card application can be lodged once he has assembled the official instructions, confirmed his EB-1 or EB-2 fit on his own qualifications, arranged the funds from a documented lawful source and obtained cross-border tax advice on the gift. What he should not do is time a resignation, a house sale or the family's move to an assumed residence date, since the official material makes the outcome subject to eligibility, admissibility and visa availability rather than to a schedule.