I've been having a blast experimenting with the GreatWall fuseki. Is there any FREE information available on how to use it, or other tengen fuseki?

I don't think that you'll find a lot on it specifically, but really it's a lesson in how to use influence.

You can make approaches to corners and abandon them, or make very wide extensions, knowing that they have backup they can run to. You can invade corners and sides and know your opponent won't make much in the center if anything. You can be very aggressive, because the ladders all favor you and your opponent may get stuck trying to live in a cramped space with nowhere reasonable to run.

In a sense, you will have to do things like that because otherwise you're behind in territory, so it's key to make use of the wall effectively.

If you want a similar experience, it's not entirely different from playing a high handicap game as black. You need to keep your opponent separated and weak so they can't consolidate their advantage.

I don't think you could find any books on the great Wall fuseki you could pay for. I only know of one book in Japanese that discusses tengen fuseki.

Bruce Wilcox's EZ Go can be purchased in digital form. It has info on the Great Wall strategy. (http://webpages.charter.net/suewilcox/EZGobook.htm)

(I realize OP was asking for free info, but I am just clarifying that there is for purchase literature out there in case others are interested.