How could I create a ScreenGui in the LocalPlayer's PlayerGui using GetPlayerFromCharacter?

Using GetPlayerFromCharacter? You would only need that if you had the character and needed the player. You didn't specify, but I'm going to assume you want to give people the GUI when they touch a part. If not, this information should still help you.

Setting up the touched event,

There are a few ways to do this. I'll be using Anonymous Functions. This script assumes the script is in the desired part.

script.Parent.Touched:connect(function(p)
    -- Code Later
end)

We get p from the touched event. P represents the object that touched the part. p can be named anything, it's just a variable.

Getting the Player,

Now we can use the GetPlayerFromCharacter function. If the parameters given aren't a character, the function will return nil. We can use an conditional statement to check for this.

script.Parent.Touched:connect(function(p)
    local plr = game.Players:GetPlayerFromCharacter(p.Parent)
    if plr then -- If plr doesn't equal nil
        -- Code Later
    end
end)

plr can equal a player or nil. The if statement checks for this to make sure there's a player.

Now we give the player the GUI,

Script, even with FE enabled, can give player's GUIs. They can't access GUIs in the PlayerGuihowever, unless the script made it.

script.Parent.Touched:connect(function(p)
    local plr = game.Players:GetPlayerFromCharacter(p.Parent)
    if plr then -- If plr doesn't equal nil
        local PGUI = plr:FindFirstChild("PlayerGui")
        local Gui = Instance.new("ScreenGui",PGUI)
    end
end)

This is just an example.

Good Luck!

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