The site template does become available in the web interface after the addtemplate operations successfully takes place. Although I have never heard of it called a "global template gallery," the globally available site templates that can be applied at the top level of any site collection on the farm (ie, not limited to a single web application), do show up in the "Select a template" list in the Create a site template page, under the custom tab.

Maybe, if you stretch it, that list can be considered a global gallery, able to display all globally available templates to use for site collections, grouped under tabs indicating whether they were built in (such as the collaboration and meeting templates), or added (for example, application templates or custom).

Regardless-- once you "addtemplate" the template you added does become available in the GUI to apply to any site you wish within the farm.

Contrary to nuclear's edit above, I do not believe there is any way to view these templates through the web interface.

When you add the template with "addtemplate", you have to do "iisreset" and then you can see the GLOBAL ID of your template with the command:

"stsadm -o enumtemplates"