Single-Object Scrolling Parallax Background System.
I designed this for the purpose of having a super simple and portable parallax implementation. With this, there's no need for managing so many messy background layers in every room. Just drop the object onto your back-most instance layer and it should handle the rest. Assign a GIF to the object, and let each frame of that GIF be one of your parallax layers. Have as many/little layers as you like, and the object should balance them accordingly.
Though a free example sprite is included, the purpose of this asset is the code. The screenshot examples are the work of an artist who's images I cannot distribute. This is not a background image for sale, this is simply an object that will manage parallax scrolling.
Works in GMS2. May need tweaking for GMS1. If you have questions I will try my best to answer them. Enjoy!