It looks like Google have changed their Drive interface and the 'Preview' button to generate a URL for the gallery's 'index.html' page is no longer displayed on the toolbar.

Please see this Google forum thread relating to this issue.

Try one of the following to get the required URL to use in your iframe:

Method #1:
(1) In a web browser, log into your Google Account.
(2) Open Drive.
(3) Make sure that you are using the old Drive by clicking on the 'cog' icon and selecting 'Leave the new Drive' (if necessary).
(4) Open your gallery folder.
(5) Select the checkbox next to the gallery's HTML index page.
(6) Make sure the 'Details/Activity' side pane is open (by clicking on the 'info' icon if necessary).
(7) Click 'Details' (rather than 'Activity).
(8) Scroll down to the 'Hosting' section.
(9) Copy the URL displayed.

This screenshot should hopefully make things a little clearer.

Method #2:
Alternatively, you could build the required URL manually as follows:
(1) Start with a base of https://googledrive.com/host/
(2) Add the Folder ID. Open the gallery folder in Drive in a browser and copy the ID from the URL in the browser's address bar. It will look something like this: 0BxXHNif2h4NRcmxHOW9lUDBWWTQ
(3) Add a slash ('/') to the URL.
(4) Add the name of the gallery's index page.
You will end up with a URL which looks like: https://googledrive.com/host/0BxXHNif2h4NRcmxHOW9lUDBWWTQ/index.html
If Google do not reinstate the 'Preview' button soon, I will ensure that the instructions on our web site are updated accordingly.

The Embedding in a Web Template Site support section has now been updated (with instructions for determining the URL to load into the iframe) for the new Google Drive.