Search Engine Roundtable reports that, Webmasters can now submit Google Sites to Google Webmaster Tools. The verification process has now been fixed, after a bug was discovered, that did not allow the verification of Google Sites for Google Webmaster Tools. Due to this bug, Webmasters weren't able to upload HTML files or add meta tags, because of which, the verification process became impossible.
There is a post at the Google Groups in regards to this announcement:
Ã¢â‚¬Å“Hi Google Sites owners!
I'm happy to announce that you can now verify and submit a Sitemap for
your Google Sites with Webmaster Tools. To verify your Site, go "Site
Settings" -> "Other Stuff" and paste in your verification meta tag in
the field labeled "Google Webmaster Tools verification."
Once you have verified your site, Google Sites auto-generates a
Sitemap XML file that you can add to your Webmaster Tools account.
For more details, check out the following Help Center article:
Google has also posted a help document, that not only guides Webmasters, as to how they can submit their Google Sites in Google Webmaster Tools, but also elaborates on the how they get their Google Sites verified in Google Webmaster Tools.