GDS counts your mortgage payment, property tax and heating cost, and sometimes half of condo fees, against your gross income. TDS adds every other debt payment on top, a car loan, a line of credit, minimum credit card payments and student loans, against that same income, which is why the total debt ceiling often binds before the housing-only ceiling does.
A CMHC minimum credit score of 600 is required for an insured mortgage on top of passing both ratios, so a thin credit file can stop an approval even when the housing and debt math looks fine on paper. Ask your lender to run both ratios against your actual numbers rather than assume a foreign rule of thumb translates directly.
GDS and TDS are the two numbers that actually decide what you qualify for here, not a rule built around a different country's lending system. Get your hand-reviewed valuation from Joel Dyck.