Technically, you should be able to use them together.  CodeSmith could generate your GAT templates (if you customize them).  That way you use CodeSmith and .NETTiers or something and modify .NETTiers templates to create the GAT templates you want.  The reason is because CodeSmith will generate the web components, BLL, and DAL, but you still won't have a decent web architecture - get CodeSmith to generate a GAT based web site instead of what .NETTies provides instead.  Plus the GAT puts in tasks that allow a user to add a page based on MVP etc....

They are complimentary, I think, but work has to be done on a template for CodeSmith.

HTH!