How to Convert this script to Be GUI Compatible?

The code you provide above is not the code responsible for actually enabling the click-on-screen function. However, I don't necessarily require it. Ensure this program below is a LocalScript, and is under the GUIButton.

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("Roka")

local Button = script.Parent

Button.MouseButton1Click:Connect(function()
	RemoteEvent:FireServer()
end)

Your code also contains a lot of poor syntax practices, so I took the liberty of providing you with a cleaner program if you wish to use it:

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("Roka")

local function OnEvent(Player)
	if (Player and Player.Character) then
		local Character = Player.Character
		if (Character:FindFirstChild("Rokakaka Fruit")) then
			local Humanoid = Character:WaitForChild("Humanoid")
			local Location = Humanoid.RootPart.Position
			Player:LoadCharacter()
			Humanoid.WalKSpeed = 16
			Humanoid.JumpPower = 50
			Character:MoveTo(Location)
			if (Player.Data.Sand.Value >1) then
				Player.Data.Sand.Value = 1
			end
		end
	end	
end