Is there a way that you can check what a user is playing on so that I could be able to move a gui so its fits the screen ?
There is a Roblox Forum thing at Devforum Roblox: Link.
Make sure to research before posting a question!
I don't know if there is a way to detect if a player is on a specific device, although you can detect if a player is using a mobile device using game:GetService('UserInputService').TouchEnabled