How to open Google Drive links directly in Google Drive app

If you open the links in the Gmail app for iOS, and you have the necessary Google Drive app installed (Google Drive, Google Docs, Google Sheets, and/or Google Slides), Gmail will by default open the links in a Google Drive app. You can enable or disable that feature in Gmail through Menu > Settings > Google Apps.

Alternatively (though it's less direct), you can paste the link into Chrome for iOS, and there will be a bar at the top of the page to open the link in a GDrive app.

Apparently you can prepend the url with the respective Google app to which it belongs. So if it is a Google Sheet, prepend the url with Googlesheets://. If it is a doc, I'm guessing you would use Googledocs://

So if the url is http://docs.google.com/spreadsheets/Google-document/, amend it with the appropriate app prefix (in this case Googlesheets:// like so: Googlesheets://http://docs.google.com/spreadsheets/Google-document.

PS: The issue is discussed at length here.