Gmail is a secured website , most any proxy will not let secure sites through. Any other website will work though. Why do u need to access gmail.com through a proxy anyway???
you can try putting in https://www.gmail.google.com or what i would do is obtain a copy of anonym.os which is a bootable cd that is linux based. it uses tor so you can access all blocked websites and has a built in instant messenger application if you need it. good luck dude, by the way i tryed the anonym.os at my school and worked perfectly if you need a iso i might be able to get you one.