Gat is an acronym for Guidance Automation Toolkit, this name is a self descriptive.
The first big example I can think about GAT usage is for all the big guidance docs in our office, every programmer wrote at least one guidance document.

For example, document that holds your team standards, such as: all of our web-pages will inherit from MyTeamMasterPage.
For example, in my team we always open new web-page which inherit from the same page, which holds some helper properties.
Another example is in our Dal & BO which both sitting in COM+ they both inherit from the same class and implement the same interfaces and so on.

This guidance docs are pretty good, because they force the same standards between the team members, but the big problem that new programmers will need more time before they could learn and use all those specific standards, and also this is a waste of time for the programmer.
When we change those standards we should learn everyone those new standards and it take some time to get used to this new standards.

The Gat is giving you whole new world to automate this process and to stop the monkey copy-paste.

For example instead of adding new empty WebForm.aspx you can cause your programmers to add the special MyTeamWebForm.aspx which holds inside the logical stuff we describe before.

I know that some of you might mumble to yourself: Merlin did the same or I wrote such Add-ins, but it is more powerful than you can think about it right now, so stay tuned for the next days I'll post here my tutorial for the Gat newbie.