As far as I can tell a gatcha is a machine like a bubble gum machne. You put your money in and one of the gumballs falls out. You don't know what color you're going to get. You put your money into the gatcha machine and one of the items falls out. You don't know which you're going to get. Here's a link to another blog post about them: http://loulouandco.over-blog.com/article-gatcha-machine-50349289.html
The prices aren't too high so you can buy several times from the same machine if you're trying for something specific. there are lots of different stores represented - I saw Miel and Reek.
The site is cute...a fun atmosphere. There are stand alone stalls along the edges of hte area and a long pond down the center with floats and rafts. The open area is covered with sand and there are some little dunes and a sand castle. Summer at the beach.
Here's a link to a review of last year's Gatcha Gatcha Fair - http://alberogssm.wordpress.com/2009/09/30/gatcha-fun/
And a link to an article from wikipedia about gatcha machines in Japan where they're called gashapon - http://en.wikipedia.org/wiki/Gashapon
Here are some pictures. Some of the gatcha machines look like actual gumball machines but there is an incredible variety.
Reek's machine looks like a red spaceship.
This stall had a cool treats vending machine
The machines in this stall looked like teddy bears.
Here - a photo booth dispenses shoes instead of snaps of you making a funny face
They had stand alone stalls and these little carts.
