How can I create a new dynamic page?

I am pretty ew to WP and I am just working on my first theme.

Now I finished the index, header and footer page, that’s fine.

But how can I now add completely custom pages? But not static ones but dynamic ones with PHP. I know I can create static pages with the WP-admin but I want to create a custom gallery page that displays the images that were uploaded with the posts (using an gallery plugin).
So I need to use PHP. How can I add a custom PHP page to the template? Or is there anywhere else I can add the code? But how do I link it to my site then?

You can see my theme here: blog.stephanskorepa.net
It’s still very basic and many parts are not finished yet, but you can see the 6 pictures at the bottom, what I want is create a gallery page that is linke do the “more” button below the images.

Hope you get what I mean ??
I’d really appreciate any kind of help!