Loop in thumbnails from the same custom post

Hey. I′m switching from categories to custom posts, and on my current page I′m looping in thumbnails from the same category as the one displaying on single.php. That is, on single.php i loop in my content and below the content comes thumbnails from the same category.

I would like to do the same thing now, but with custom post instead. I′m not much of a coder, so I dont know really how to change my current code to check for the same custom posts instead. Maybe someone can help me with this?

My code looks like this now:
https://pastebin.com/mnfGQitC

I just want to change it so it looks for posts in the same custom post! Hoping for some helpful soul out there ??