Every response tells you where you stand:
429 rate_limited with a Retry-After header in seconds. Wait that long and retry; do not retry in a tight loop.
To stay well inside the limits, use webhooks instead of polling, use updated_since to fetch only what changed, ask for limit=100 per page, and send stock in batches of up to 500 SKUs.
Need more for a large catalogue or a one-off migration? Contact BrandGate support before you start.