Custom Admins commands dont work?

Your script does not work because on line 6, you are attempting to check if the player's name is specifically "Admins", and not checking from the table properly. The for loop you have on line 5 is correct.

You are also trying to enable a GUI which is in the StarterGui and not in the person's PlayerGui itself. Doing this from a server script will not work either way, we'll need to use Remote Events to communicate between the Server and Client to make the GUI visible.

You'll need 2 scripts. One local script, and one server script. Put a localscript anywhere within the client, like the StarterGui for example.

Put the server script in ServerScriptService.

Server Script:

The v in for i, v in pairs(Admins) do signifies something inside the table while the i tells you the position the loop is currently at.

local Admins = {"jayfig04"} 

local Event = Instance.new("RemoteEvent") -- Creates a remote event
Event.Parent = game.ReplicatedStorage -- Parents it to the ReplicatedStorage
Event.Name = "GuiPanel" -- Names the remote event. You can have multiple of these with different names for different tasks.

game.Players.PlayerAdded:Connect(function(Player)
    Player.Chatted:Connect(function(message)
        for i, v in pairs(Admins) do
            if Player.Name == v then
                if message == "/open" then -- Checks the message.
                    Event:FireClient(Player) -- Fires the event at the client (in this case the admin)
                end
            end
        end
    end)
end)

Local Script:

local Event = game.ReplicatedStorage:WaitForChild("GuiPanel") -- Waits for the Remote Event, in case it takes a while to load.

local Player = game.Players.LocalPlayer -- Getting the client.

local PlayerGui = Player:WaitForChild("PlayerGui") -- Getting the client's PlayerGui.

Event.OnClientEvent:Connect(function() -- When the RemoteEvent fires
	PlayerGui:FindFirstChild("Panel").Enabled = true -- Enables your GUI.
end)

If you have any more questions, or something doesn't work, let me know by replying with a comment. If you are satisfied with the answer, be sure to accept it))

Edits:

There were three issues with the server script i provided. I accidentally added a random hyphen, which errored the script.

The other two issues were something that i didn't see that came from the original script you've provided. You were trying to use gsub without passing a string in the first argument and the other is that you shouldn't use a function value as an argument.