I have completed steps of authorization and obtained access token and refresh token.
What should I do next to generate access token using refresh token that I have stored through google drive API?
I won't be able to use any sdk since I am working on Force.com so please suggest the way to implement it directly through the API.
If you want to implement that yourself, the OAuth 2.0 flow for Web Server Applications is documented at https://developers.google.com/accounts/docs/OAuth2WebServer, in particular you should check the section about using a refresh token:
https://developers.google.com/accounts/docs/OAuth2WebServer#refresh