Redirecting folder index to common index file

Hi,

I'm currently moving my webpage from lighttpd to nginx. Everything is fine except for my gallery. My gallery currently uses the following directory structure
root
- gallery.php
- images1/
- image1.png
- image2.png
- images2/
[etc.]

Now I need the index file of any directory to be gallery.php out of the root directory. Using lighttpd I used this config:
$HTTP["url"] =~ "^/.+($|/)" {
index-file.names = ("../gallery.php")
}

Using nginx I've been trying the following, which does not work:
location = "/.*($|/)" {
index /genthumb.php;
}

This only gives me an error in nginx' error log:
2013/02/05 23:42:12 [error] 4004#0: *1 directory index of "/var/www-pub/gallery/" is forbidden, client: 217.231.250.210, server: gallery-dev.*, request: "GET / HTTP/1.1", host: "gallery-dev.mydomain.com"

Is there any simple way to fix this issue, or do I need to copy my gallery.php around?