Will Gallery3 be compatible with Windows/IIS? I didn't see that in the documents.
Scope #2, Supported Configurations - Gallery 3 is supported on Linux / Unix servers, running a MySQL 5 and an Apache 2.2 web server with PHP 5.2. Emphasis on supported, not necessarily required. It may well work with MySQL 4.1 on MS Windows as well. But the Gallery team is going to focus its energy on making the best possible product on the supported configurations.
....the gallery dev team is not going to invest time into supporting Windows. But this is open source, so if it's of enough interest to the community I'm sure you'll see some support emerging.
Blog & G2 || floridave - Gallery Team