Please see if this helps: http://salesforce.stackexchange.com/questions/8538/convert-time-from-a-different-timezone-other-than-local-to-gmt
Or you can also use the GMT related methods in the datetime class: https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_datetime.htm
Or you can also use the GMT related methods in the datetime class: https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_datetime.htm