Installation is simple.

First, place all the gallery files included in this distribution into
a directory on your website where you would like the gallery to be based.
Then create a directory within which you would like to upload your images.
Within this directory, create a directory for each category (directories may
have spaces in the name) and fill each category with images.

Create a database and a user to access it, and fill in the details in
gallery.ini.

Go to index.php just as you would if you were to view the gallery. The
script will take a few moments on the first run to thumbnail and store all
the various images, and then it will display the gallery index page.

You're done.

Enjoy,
Richard.