Hi,
Editing web.config is something you could do with .NET APIs (and not GAX), please take a look at:
http://www.odetocode.com/Articles/418.aspx
Once you have the code written that updates web.config you can put that code into a GAX Action and call that code from a GAX recipe, but the code on how to play with web.config is not related to GAX.
HTH,
-vga.