Automatically add class-attribute to gallery?

Question

I'm making a theme for a client in WordPress and he wants each gallery to have a grouped lightbox, so there's a lightbox for each gallery. I found out you can make it add an attribute to the gallery automatically, so what I want is an attribute called 'class' that has a number that adds by 1 for each gallery created. How would I do that?

Edit:
I see that I was not being specific enough so here's some more information: I want a 'class=""' attribute added automatically to the shortcode when the post gets created. Inside that class attribute I want it to have a number that goes up by 1 for each post containing a gallery. So the shortcode would end up like this: [gallery class="1" ...] and the next [gallery class="2" ...] and so on...

Answer 1

If you take a look at native galleries they already have IDs with counter implemented:

$instance++;
...
$selector = "gallery-{$instance}";
...
$gallery_div = "<div id='$selector'class='gallery...

So first gallery within a post will have <div id="gallery-1"... and so on. This is used by core itself to target different styles at each instance.

If you need to do something more custom you can use gallery_style filter, which gets passed those styles and opening div markup, to similarly keep track of instances and inject your custom classes as necessary.