"PlayerGui" is not a valid member of "Part?" [SOLVED]

On line 28, you can attempting to call PlayerGui within the Model/BasePart type instance, NOT the Player itself. This can be fixed, however, by using the GetPlayerFromCharacter method, also, on lines 6-26, and lines 29-40, that amount of code is not necessary, why not use a For loop to loop through all the Children within the Parent [within 'jumpp' and 'dooropen']? However, there is still three problems remaining; 1. What happens if the function fires multiple times 'onTouch'? 2. What if the 'BasePart' type instance that touches the 'BasePart' type instance is not a 'Player'? 3. Why is line 5 necessary? These problems, however, can be fixed, by checking if the thing touching it is indeed a 'Player', by using a 'Debounce' [to keep the function from firing multiple times], but however, I do not know what line 5 is necessary for, but, with all the fixes I have displayed, we can not recode, and fix your code;

local Debounce = false --This is our 'Debounce' now; This will prevent the function from firing multiple times 'onTouch'
local jumpp = game.Workspace:WaitForChild("jumpp"); --Just realizing, why not use the 'WaitForChild' method? Because,  what if 'jumpp' is not existant at the time of execution?
local dooropen = game.Workspace:WaitForChild("Dooropen"); --Same problem that 'jumpp' had, which if 'Dooropen' was not existant at the time of execution?

script.Parent.Touched:connect(function(hit)
if Debounce or not game.Players:GetPlayerFromCharacter(hit.Parent) then return false end --Here is our 'Debounce' now; it will check to see if 'Debounce' is equal to 'true', and if it is, it will prevent the function from firing again, or, from firing multiple times 'onTouch', lastly, our method to get the 'Player', the 'GetPlayerFromCharacter event'! The 'if' statement will check to see if the current instance touching it is a 'Player'
local player = game.Players:GetPlayerFromCharacter(hit.Parent) --Variable/Identifier 'player' is now identifying 'player' as the 'Player' touching the Brick/Part
Debounce = true --Reverts 'Debounce' to 'true', starting the prevention of the code from firing multiple times at once
for i,v in pairs(jumpp:GetChildren()) do --Here is our 'for' loop now; It will loop through all the current 'Children' within 'jumpp'
if v:IsA("BasePart") then --What if the current child [v] was not a 'BasePart' type instance? This will check to see if the current Child [v] is a 'BasePart' type instance
v.Transparency = 0 --Will now revert the 'BasePart' type instances 'Transparency' property to '0', allow the 'Player' to see it
end
end
game.Lighting.scare:Clone().Parent = player.PlayerGui --Instead of reverting the original's Parent to the Player's 'PlayerGui', why not 'Clone' it, then revert the 'Cloned' version into the 'PlayerGui'?
for i,v in pairs(dooropen:GetChildren()) do
if v:IsA("BasePart") then
v.Transparency = 1
v.CanCollide = false
end
end
wait(2) --Waits '2' seconds before going on
Debounce = false --Reverts 'Debounce' to 'false' again, allowing a 'Player' to use the function again
end)

Hope this helped!

You forgot that the argument for Touched is the part that touches. So basically you are trying to go into whatever part that touched it and find PlayerGui inside of said part. Also, you need to go to the Player, not the Character. You want to go to it's Parent then use GetPlayerFromCharacter() which does what the name says. Here's your script, if it doesn't work then just comment and I'll fix my error!

local jumpp = game.Workspace.jumpp
local dooropen = game.Workspace.Dooropen

script.Parent.Touched:connect(function(player)
    script.Disabled = true
    jumpp.Head.Transparency = 0
    jumpp.LeftArm.Transparency = 0
    jumpp.LeftArmb.Transparency = 0
    jumpp.LeftArmc.Transparency = 0
    jumpp.LeftArmd.Transparency = 0
    jumpp.RightArm.Transparency = 0
    jumpp.RightArmb.Transparency = 0
    jumpp.RightArmc.Transparency = 0
    jumpp.RightArmd.Transparency = 0
    jumpp.LeftLeg.Transparency = 0
    jumpp.LeftLegb.Transparency = 0
    jumpp.LeftLegc.Transparency = 0
    jumpp.LeftLegd.Transparency = 0
    jumpp.RightLeg.Transparency = 0
    jumpp.RightLegb.Transparency = 0
    jumpp.RightLegc.Transparency = 0
    jumpp.RightLegd.Transparency = 0
    jumpp.Torso.Transparency = 0
    jumpp.Torsob.Transparency = 0
    jumpp.Torsoc.Transparency = 0
    jumpp.Torsod.Transparency = 0
    wait(.1)
    game.Lighting.scare.Parent = player.Parent:GetPlayerFromCharacter().PlayerGui
    dooropen.Window.Transparency = 1
    dooropen.Windowinv.Transparency = 0
    dooropen.Windowframe.Transparency = 1
    dooropen.Windowframeinv.Transparency = 0
    dooropen.DoorBottom.Transparency = 1
    dooropen.DoorBottominv.Transparency = 0
    dooropen.Window.CanCollide = false
    dooropen.Windowinv.CanCollide = true
    dooropen.Windowframe.CanCollide = false
    dooropen.Windowframeinv.CanCollide = true
    dooropen.DoorBottom.CanCollide = false
    dooropen.DoorBottominv.CanCollide = true
    jumpp:Destroy()
end)
--This is all written in a Regular Script.