The new feature is only available with the new Compose editor
The integration between Gmail and Google Drive is getting deeper as it is now possible to attach Drive files to messages without leaving the email app.Along with the regular attachment, photo or link, there's now a Google Drive icon in the Gmail compose window that, you guessed it, enables you to add a file from your Drive folder.
"With Drive, you can insert files up to 10GB -- 400 times larger than what you can send as a traditional attachment. Also, because you’re sending a file stored in the cloud, all your recipients will have access to the same, most-up-to-date version," Gmail explained.
The feature is only available to Gmail users that have switched over to the new Compose editor. What's more, it's rolling out over the next few days so, chances are, you don't have it just yet.