Often we get questions about the architecture of the Gallery website. Hopefully this page can answer most of your questions, please add them to the talk page here and we'll get the information added.
The Gallery website, which we refer to as GMC (Gallery.Menalto.Com) is composed of several components. There's the main Drupal install at galleryproject.org, our Gallery installation at galleryproject.org/gallery/, and our documentation site running MediaWiki at codex.galleryproject.org. Some information about how we maintain these is available on the Website Maintenance Processes page. The main thing tying these components together is our custom themes for each piece. You can read more about the beginnings of the "new" Gallery website in the news story about the launch: Welcome to the new Gallery website!
For system software, we are typically using the most recent stable Apache 2, MySQL, PHP, and eAccelerator in Debian on a Dual Opteron server with 4GB of RAM.
Our Drupal install is mostly stock components with a few modifications.
Pretty much a stock MediaWiki install with a custom theme and the gallery2wiki extension. This site looks mostly like GMC, but requires a separate account. (Like the Drupal theme, Sorry, we can't give you our theme. We put a lot of work into it and it's important that the look of our site is identifiable with us.)