How Can I make a ScreenGui appear when the mouse is hovering over a part?

You could place SurfaceGui's around the part, and when it's hovered, find the closest player.

You can keep the screengui in playergui if you want. The "Player" parameter tells you which player clicked it. We can then go to the Players gui by finding the players name. (I'm not sure if you can just do "Player.PlayerGui", you can try it out.

You will need to change the script depending on the screengui name and stuff

script.Parent.ClickDetector.MouseHoverEnter:connect(function(Player)
    game.Players:FindFirstChild(Player.Name).PlayerGui.ScreenGui.Frame.Visible = true
end)

script.Parent.ClickDetector.MouseHoverLeave:connect(function(Player)
    game.Players:FindFirstChild(Player.Name).PlayerGui.ScreenGui.Frame.Visible = false
end)

Just make a GUI and put it in the Lighting and create a part and insert click detector and script in it and write a script showing that when a player hovers the mouse over the part it creates a clone of that gui from lighting and place it in playerGUI.

script.Parent.ClickDetector.MouseHoverEnter:connect(function()
    game.Lighting.ScreenGui:clone().Parent=game.Players.LocalPlayer.PlayerGui
end)

script.Parent.ClickDetector.MouseHoverLeave:connect(function()
    game.Players.LocalPlayer.PlayerGui.ScreenGui:Destroy()
end)