I'm making a game, and I want to optimize GUI experience for both Mobile and Laptop/PC players. If I have a Mobile list of GUIs that I copy to the replicated storage if the player is on a Mobile device and a Laptop/PC list of GUIs that I copy if the player is on a Laptop or PC, will those GUIs be immediately accessible to the client when copied? (Of course I could test this out on my own, but I'm not yet to this point of development so it would save me some time if I were informed now rather than later.)
All contents of ReplicatedStorage are loaded on the client when they join the server. This would be no different as if all the GUIs were in StarterGui but not visible until a LocalScript has determined what device they have.
If you want the GUIs to be immediately accessible when given to the player, then you don't have to do anything. However, if you have images that you want loaded as fast as possible for the player, try preloading.