If the Attach button doesn't appear on the Gdrive tab your NetSuite Admin will need to add full permission(s) to the NetSuite role your user is using (on the Custom Record subtab)  for the following records.

Step 1: Navigate to Setup > Users/Roles > Manage Roles.

Step 2: Locate the role you want to modify. Click the corresponding Edit or Customize link.

Step 3: Go to Permissions> Custom Record and add the following records:

Step 2: Go to Permissions> Lists and add Document and Files.