Using gmail with postfix smtp server
I'm currently running an Ubuntu 14.04 VPS with postfix set up by following this tutorial. It's working great and all, but I want to primarily access my e-mail on this server using gmail.
Basically I want my server to forward all e-mail to my gmail, but I also want gmail to be able to send mail using my server, using the setting on gmail under "Accounts and Import"/"Send Mail As:"
Mainly I need to be able to fill out this form.
I'm a little confused as to which tutorial I should be using for this.