Running user made php files

I apologize for this extremely dumb question but I am a PHP noob and have somehow been able to confuse myself to the point of almost no return.

I have a php file (let’s call it gallery.php) that I have created by taking the index.php and removing the “post loop” and replacing it with other PHP code for a plugin I know for a fact works. I have the gallery.php file in my themes folder (I’m using the 2/8/2005 nightly) but when I type in the address for gallery.php to see what the output looks like, I get this error:

Fatal error: Call to undefined function: get_header() in /home/.unger/spyderfcs/spyderfcs.com/wp-content/themes/elektro/gallery.php on line 1

Now, my question is how do I get the gallery.php to run and display the proper html? Does it have to be in the base wordpress directory and not the themes directory? I know it’s a dumb question but any help would be great and I’d be very thankful.