How would I get the magnitude between an npc and a player?

So I just created a script that opens a gui when the player comes close to a part. I just did the part for an initial test but I don't know how i'll adjust it for npc's. I was thinking to create a ray that detects if an npc is in it's vicinity but I don't know how to go about making that vicinity and I don't how to do it with multiple npc's at once.

LocalScript in StarterPack in a tool

local playergui = game.Players.LocalPlayer.PlayerGui
gui = Instance.new("ScreenGui", playergui)
    image = Instance.new("ImageLabel", gui)
    image.BackgroundColor3 = Color3.new(255, 255, 255)
    image.BorderColor3 = Color3.new(27, 42, 53)
    image.Position = UDim2.new(0.46, 0,0.423, 0)
    image.Size = UDim2.new(0, 100,0, 100)
    image.ImageColor3 = Color3.new(255, 255, 255)

while wait(.01) do

local mag = (game.Workspace.Part.Position - game.Players.LocalPlayer.Character.HumanoidRootPart.Position).magnitude
local playergui = game.Players.LocalPlayer.PlayerGui
local val = 10

if mag < val then
	    print("gfsdgsdfgsdfgsdfg")
	    gui.Parent = playergui
else
	    gui.Parent = nil
    end
end