****for example I click a button and that button make a textlabel say Hello but, it only appears for me not for everyone else.**
Please give us the script so we can figure It out, but try using a RemoteEvent
RemoteEvent Information: https://developer.roblox.com/en-us/api-reference/class/RemoteEvent
By your example I undersrand that you want to change a textlabel using a script for everyone, right? If yes, then it is possible but you have to use :GetPlayers()
and do for every player in server the same thing to gui. Or you can use a local script and a script to do that (By firing the event for all clients). Bonus: ServerScripts don't change the gui for everyone by just changing one player's gui poperty