How do I make my AboutFrame visible when I click the AboutBtn?

EDIT: To disable every button, you can get every descendant of the player's PlayerGui using GetDescendants() and check if it's an ImageButton or a TextButton using IsA("GuiButton"). To get the PlayerGui from the script, you can use script:FindFirstAncestorOfClass("PlayerGui").

-- LocalScript inside AboutBtn
local AboutBtn = script.Parent
local AboutFrame = AboutBtn:WaitForChild("AboutFrame")
local PlayerGui = script:FindFirstAncestorOfClass("PlayerGui")
repeat -- if FindFirstAncestorOfClass returned nil/nothing (failed)
    PlayerGui = script:FindFirstAncestorOfClass("PlayerGui") -- we will define it again
    task.wait() -- so game doesn't crash/lag
until PlayerGui -- until we got the PlayerGui (success)

local debounce = true -- we will use a debounce so that we can give time for it to visible/invisible all buttons
AboutBtn.MouseButton1Click:Connect(function()
    if debounce then
        debounce = false
        AboutFrame.Visible = not AboutFrame.Visible
        for _, descendant in ipairs(PlayerGui:GetDescendants()) do
            if descendant:IsA("GuiButton") and descendant ~= AboutBtn then -- if descendant is an ImageButton/TextLabel and not AboutBtn
                descendant.Visible = AboutFrame.Visible
            end
        end
        debounce = true
    end
end)

OLD: @666_brithday didn't properly explained his answer, so this answer is an improved version of his answer.

MouseButton1Click fires when the button was clicked and released using the left mouse button. This is a combination of MouseButton1Down (when the player clicks the button but haven't release the left mouse button) and MouseButton1Up (when the player releases the left mouse button in the button after holding the left mouse button). This event also works in mobile devices.

An alternative is Activated, which is like MouseButton1Click and MouseButton2Click (right mouse button), except that it can work in any UserInputType other than the left and right mouse button, such as MouseButton3 (pressing the mouse wheel), MouseWheel (scrolling the mouse wheel), and Touch (pressing the mobile screen).

And I also fixed his script:

-- LocalScript inside AboutBtn
local AboutBtn = script.Parent
local AboutFrame = AboutBtn:WaitForChild("AboutFrame")

AboutBtn.MouseButton1Click:Connect(function() 
    AboutFrame.Visible = not AboutFrame.Visible 
end)

try this

local AboutFrame = script.Parent --Keep adding .Parents until you reach the AboutFrame 
local AboutBtn = script.Parent --Keep adding .Parents until you reach the AboutBtn

AboutBtn.MouseButton1Click:Connect(function() 
AboutFrame.Visible = not AboutFrame.Visible 
end)