Via: header is added by proxies, both forward and reverse, and can appear in the request headers and the reply headers. That field containing "GWA" is notionally the hostname or pseudonym field, possibly an identifier for the proxy/firewall, e.g. "gateway A", :
Via = "Via" ":" 1#( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
received-by = ( host [ ":" port ] ) | pseudonym
Since you're seeing that in the request headers (presumably on the web server itself), and you're not sending it, it must be a server side reverse-proxy/accelerator. My best guess is Google PageSpeed, you should be able to confirm it by checking the DNS CNAME for the web site, PageSpeed uses names matching
*-ps.googleusercontent.com for content caching. I cannot confirm its behaviour with the
Via header though, that could be added at any step on the way.
You may be able to bypass the PageSpeed servers to confirm (or deny) if they add it by using something like:
wget -S http://126.96.36.199/ --header "Host: www.website.com"
Where 188.8.131.52 is the real public IP of the hosted website (i.e. not a PageSpeed server), and www.website.com is the real address.