The Microsoft Web App Gallery uses a live freed from Microsoft's service at the link below;
If you do not find the application or version you want in the list it is because the developers of the app have not released a Web App Gallery version that they have made available through Microsoft's service.
With applications like Umbraco all Web App Gallery does is download the application and unzip the files to the websites wwwroot folder.
You can do the same by uploading the applications files yourself via FTP to your websites wwwroot folder.
Not really sure what you mean by requiring root access, but upgrading to new versions would not be done through the Web App Gallery service.
Many applications have tools within their online admin area that allow them to self update at a click of a button. For these types of updates to work you need to enable write permissions on the website, but for security it is available to disable write permissions once the application is updated.