It's possible but there are some issues.
1. Create a forum then a forum and gallery sub-forums under the top level.
2. Visit each of the secondary applications a select: Options > Embedding options.
(This will provide the code you need to embed each in the two pages of your main site.)
This structure will allow a single login that gains access to both the embedded applications. Now we come to the problems...
Each of these applications, will show the top level (root) forum in the breadcrumb menu at the top of each application. That means it becomes possible to to navigate from the forum to gallery and back, while the parent page on your main site thinks it is still showing the original application that was embedded there.
So you also need to edit the NAML code to remove the parent application from the breadcrumb menu that appears on each application.
Assuming that you also place a redirect on each application, so visitors can't break out of your main site, removing the root application from the bread crumbs also means you need to have a "back door" available so you can get to it, otherwise you are unable to undertake certain forum management tasks because certain options are only available at the root level Options menu. That means you need an additional page on your main site that has the root application embedded in it. The page should not be linked from anywhere in your main site. Only you need to know its URL.
There are further issues. When a user logs out Nabble always leaves them at the root level so should they login again they are taken to the root application. It is also possible to reach the root forum via the "Edit this page" link. Overcoming these require further edits to the standard NAML code.
I have a site with a similar structure (MIxed and News applications) embedded in it and Pedro has been able to provide the necessary edits for me. If you visit:
http://www.rustonreaches.plus.com/you'll see the pages "Forum" and "Blog" and if competent with NAML you should be able to work out what edits need to be made in your own site. Otherwise you'll probably need to pay for Premium Support to get it done for you, as I suspect it's too much to expect such heavy customisation to be done as a free service. (Indeed, I don't even have an answer installed for the "Edit this page" link, so it is possible on my site for the technically curious to access the root application and confuse the main site navigation. I must talk to Pedro about this!)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.