I have a screen GUI that needs to be shown. But the problem is that I am running the program instead of playing the game as a player. Is there a way I can show the ScreenGui while I “Run” the game?
I tried using ScreenGui.Parent = game.CoreGui but it didn’t work. 
I’m pretty sure you can just click on the screengui, and on properties just click enabled, assuming that it is inside the playergui.
But player Gui only works in game as a player. Running the game acts as a “Server Sided” play. This means it doesn’t show. There is no player to show the gui to if it is in PlayerGui.
What do you mean? there has to be a player
If you run the game and not play it, you do not spawn as a player. The reason the folder is called “PlayerGui” is because it is local to the player only. If it wasn’t then everybody could see the same ui.
Have you tried it? I just tried it myself and it works even when I’m only “running” the game. (serverside). And if you mean actually using the gui, I don’t think that would work serverside.
Are you trying to get some kind of Plugin behavior right here? Since as far as I know the CoreGui needs a player too, if not, then it shouldn’t have anything to show, right? I may be wrong, but stills my question, is this for a Plugin?
Edit: So it seems that I was kind of “right”, since the CoreGui service it is the one that is in charge of creating the GUI elements via in-game for the core-user interface.
Yes, but this is not for a plugin. I don’t know why this works on a plugin and not here.
If you’re trying to do this server-side within the Run test mode of studio, you can’t show guis to my knowledge. Your best bet would be using a plugin and there’s plenty of functions within RunService you can use to determine if it’s the server and what not.