Is an API Gateway sufficient as a standalone DDoS protection solution?

Generally, no. While API Gateways provide valuable DDoS mitigation capabilities, they are rarely sufficient as a standalone solution, especially against large-scale, sophisticated attacks. They are best used as part of a layered security approach, combined with dedicated DDoS protection services (like those offered by Cloudflare, Akamai, or AWS Shield) and a Web Application Firewall (WAF).