not used an iPad but have used gdrive with android so it should be fairly similar
you need to register with google to get oauth set up there are plenty of instructions around for this
once that is set up a file upload requester should have an extra item gdrive
selecting that will show a button labeled login
on android I got a list of various apps i could upload from i chose google drive.
it then got access to my files which i was able to select and upload to the site.
So essentially there are two parts set up your server so it will work with gdrive and installing the gdrive app on your portable device.
hope that helps.