Would this work???

I'm still having a little trouble with tables since 2 weeks, I'm not sure if this script will work:

player = game.Players.LocalPlayer
a = {36491121, 57099125, 60788697}
if player.userId == #a then
	script.Parent.Parent:Destroy() --This is a ScreenGui
end

What it's meant to do is if the player's userId is one of those in the table, the GUI permanently gets removed from his StarterGui.

You've misunderstood tables a little bit.

The # means "length." So, #a is just going to be 3. With that in mind, your read reads:

player = game.Players.LocalPlayer
a = {36491121, 57099125, 60788697}
if player.userId == 3 then
    		script.Parent.Parent:Destroy() --This is a ScreenGui
end

Naturally you aren't trying to check whether their player ID is 3. So here's what you can do instead: iterate through the table.

Start with setup:

player = game.Players.LocalPlayer
a = {36491121, 57099125, 60788697}

Next, we need some way to tell whether they are in the list.

areTheyInTheList = false

It starts false, because we haven't found them in the list yet.

Now, we will look through the list:

for i = 1, #a do

What this means is that i will first be 1, then be 2, then be 3, and stop at that point, since #a = 3.

Inside this list, we need to look at a[i] and check it. So:

	if a[i] == player.userId then
		areTheyInTheList = true
	end

Once we find it, we set it to true. Then, end the loop:

end

And now we act on it:

if areTheyInTheList then
	player:Kick()
end

All together, our script is:

player = game.Players.LocalPlayer
a = {36491121, 57099125, 60788697}

areTheyInTheList = false

for i = 1, #a do

	if a[i] == player.userId then
		areTheyInTheList = true
	end
end

if areTheyInTheList then
	player:Kick()
end

a = {36491121, 57099125, 60788697}
if player.userId == #a then
    script.Parent.Parent:Destroy()
end

I can see several things wrong with this script that would break it entirely. Where did you define player? IS the script.Parent.Parent the player? You need to use a loop to go through that table. Why not use the :Kick() method? Is this script in StarterPack or StarterGui?

Here's my modified version of your script;

--THIS MUST BE IN A LOCAL SCRIPT IN STARTERGUI OR STARTERPACK!
player= game.Players.LocalPlayer
a = {36491121, 57099125, 60788697}
for _,v in pairs(a) do
if v== player.userId then
player:Kick()
end
end