There are lot of free proxies available ,but as per google docs we need to add a custom header to every google request ( X-GoogApps-Allowed-Domains) to block consumer account
Block access to consumer accounts - Google Apps Help
The best simple method is using one small proxy called burp suit download it from its free version is enough for doing this
Download Burp Suite 1.5- Burp Suite is an integrated platform for performing security testing of web... - SPIDERSOFT - Download Free Softwares and drivers
the detailed installation and configuration is found
COMPUTECH » Block access to consumer gmail accounts but allow google apps
in a small organisation its better to use and effective