When the GC will collect the data, like images and all? or is there any doc on lightning GC?

If the images goes out the screen the GC will collect the data or once the gc threshold limit reaches only ?
I want that once the images goes out of the screen they should get removed from the memory or GPU. How can i do that?

You can force a garbage collection at any time via this.stage.gc(). I recommend you experiment when the best time to call this method is and not call it too frequently as it does require CPU cycles to collect garbage. Usually its best to call this when pages transition.