I've been working on a universal game with SpriteKit, and I have my images in .gif I have the normal definition the @2x and the @3x, I need to know how to create an atlas and how to change between atlases when the device is bigger or smaller and which of the definitions (@2x, @3x) is for what device
Xcode will automatically create an atlas for you. Use the Images.xcassets to store your images at @1x, @2x & @3x. Xcode will auto pick the appropriate image resolution for you.
You need to read Apple's Asset Catalogs docs.