Step 1: Open Gmail in your browser, then click Compose to create a new message.
Step 2: Near the bottom of the new-mail window, locate and click the Google Drive icon.
Step 3: Now you'll see the contents of your Google Drive. Choose one or more files, then select the Attachment option in the bottom-right corner. At first glance it may not look like anything has happened, but basically you're just toggling between that and the Drive Link option.
Step 4: Click the blue Insert button (bottom-left corner), then finish sending your message as usual.