I'm not too familiar with G-WAN, but there's many more factors than some benchmarks published by the author of G-WAN: 1) G-WAN is closed source. This means you

I'm not too familiar with G-WAN, but there's many more factors than some benchmarks published by the author of G-WAN: 1) G-WAN is closed source. This means you're tied to their paid support for fixes and improvements. Web servers are pretty critical pieces of infrastructure that are not replaced easily (if you're doing large deployments, custom config, etc.) and so having this risk is huge. 2) Documentation, configurability, support for well-tested integration with various application servers. For example, Nginx supports uWSGI out-of-box. The G-WAN site seems to have limited documentation. 3) Nginx is well proven in production. Sometimes just being incumbent is a good reason. This often takes some large project or company taking a risk on the platform, proving it in the process; this is unlikely due to #1. 4) Other benchmarks indicate otherwise in regards to G-WANs performance; not to say its bad, but the selection of benchmarks on its site is likely biased towards it. There may be many other reasons...