I am not using Google Drive myself (I am using a few different competitors' storage tools that similar in design), but here are my thoughts in case some of them would be helpful:
1. It should be the responsibility of the cloud-storage-drive app to provide the link for the file (where sharing by link is available/enabled). Other apps would not (and should not!) be able to generate such a link.
I would expect Google Drive app to have an option of sharing a file by the link, where you would choose the file, and could share it by the link with any other app, including private messaging apps (Skype, Viber, Whatsup, ...) and e-mail apps. If it doesn't, - that's a shortcoming of that app.
2. Gmail has some additional integration with Google Drive, but I doubt they have similarly extensive integration with all other cloud-storage solutions (Dropbox, box.com, Amazon Cloud, mega, jumpshare, Yandex Disk, One Drive, pCloud, Mediafire, Sync.com, Carbonite, etc.
Would you expect Aquamail (or any other e-mail app) to be that universally integrated?