How do I make this work on a SurfaceGui and repeat it over and over again?

You have a logic problem in the PlayStory function: you fade out the text, replace the story (but over a long period of time; adding one character at a time to the invisible label) before finally fading in the entire story.

By "repeat it over and over again", I will assume you mean "how do I loop the story so that it plays indefinitely".

One option is to just put the PlayStory function (and the nearby 'if' statements) in a while loop that goes on forever (with a wait(1) command to prevent freezing).

If you want the story to only run when the "Play" boolean value is true, then connect the function to a Changed property (that is, put lines 43+ in a function that is called whenever "Play" changes):

val.Changed:connect(function()
	if val.Value == true then
	    val.Value = false
	    wait(1)
	    PlayStory(script.Parent.Story, script.Parent, story)
		wait(1)
	    script.Parent.Visible = false
	end
end)

Alternatively, there's the method where you show one sentence at a time (and let each fade out/in).

Here is the script for this alternative method:

print("Intro is loaded!")
local credits = "" -- Names of robloxians who worked on your game
local story = "Enjoy the game and buy whatever you like. Be sure to comment for any more ideas or anything you want to sell. Give it a thumbs up and/or fav it. Enjoy!" -- Intro story.
function PlayStory(label, frame, segment)
    for i = 0, 1, 0.1 do
        frame.Title.TextTransparency = i
        frame.Title.TextStrokeTransparency = i
        label.TextTransparency = i
        label.TextStrokeTransparency = i
        wait(0.1)
    end

    label.Text = ""
    label.TextXAlignment = "Left"
    label.TextYAlignment = "Top"
    label.Size = UDim2.new(1, 0, 1, 0)
    for i = 1, 0, -0.1 do
        label.TextTransparency = i
        label.TextStrokeTransparency = i
        wait(0.01)
    end
	label.Text = segment
    for i = 0, 1, 0.1 do
        frame.BackgroundTransparency = i
        label.TextTransparency = i
        label.TextStrokeTransparency = i
        wait(0.1)
    end
end

while true do
	for segment in string.gmatch(story, "[^.!?]*[.!?]*") do
	    PlayStory(script.Parent.Story, script.Parent, story)
	    wait(4)
	end
end

To make string.gmatch match an entire sentence, we use this: string.gmatch(story, "[^.!?][.!?]") The [^.!?] means "any character not a period, exclamation mark, or question mark" (the ^ means "not any of the following characters"), the * means "as many of these as possible", and then the rest says "as many punctuation marks as possible".