Impossible GUI to create?

The main problems: MouseButton1Click doesn't return any parameters, hit is mainly for touched events; putting the grav coil clone into StarterGear requires the player to die before obtaining the gear, which I assume to be undesired; if the MouseClick function worked, by calling the find function, the player will receive the gear twice.

local ID = 158694379 --Local variables are generally better practice because they allow for faster lookup and limit variable clutter between functions
local badge = game:GetService("BadgeService")
local player = game.Players.LocalPlayer --an alternative for getting the player only in local scripts

function find(userid, badgeid)
    if badge :UserHasBadge(userid, badgeid) then
        return true --replaces wherever the function was called with the bool "true"
    else return false --same as above with "false
    end
end

function MouseClick()
    --if (hit.Parent:FindFirstChild("Humanoid") ~= nil) then --this line and the two following lines are unnecessary when using a gui
    --local p = game.Players:GetPlayerFromCharacter(hit.Parent)
    --if (p ~= nil) then --and if you are using this, then "if p then" is fine. If p is nil, it'll return nil or false and not run the code under that if statement.
    if (find(player.userID, ID)) then
        local clone = game.ReplicatedStorage.GravityCoil:clone() --ReplicatedStorage was designed to replace lighting as a storage box for items that need to be accessed by a client. I suggest putting the gravity coil there.
        local clone2 = game.ReplicatedStorage.GravityCoil:clone()
        clone.Parent = player.Backpack --adding it to the backpack and StarterGear gives the player the gear immediately and allows the player to keep it every time they die. By omitting the backpack in your original code, you require the player to die before obtaining the gear. 
        clone2.Parent = player.StarterGear
    else
        script.Parent.MouseButton1Click:Destroy() --remove() is depreciated and I took out p
        --script.Parent.BackgroundColor3 = Color3.New(.5, .5, .5) --The color is intended to be a darker shade of the normal background color of the button. This is suggested so that the player will know what they can get, and to reduce the amount of work required to make sure that the guis appear as intended.
    end
end

script.Parent.MouseButton1Click:connect(MouseClick)

Or, condensed to one function (minus comments)

local ID = 158694379
local badge = game:GetService("BadgeService")
local player = game.Players.LocalPlayer

function find(userid, badgeid)
    if badge :UserHasBadge(userid, badgeid) then
        local clone = game.ReplicatedStorage.GravityCoil:clone()
        local clone2 = game.ReplicatedStorage.GravityCoil:clone()
        clone.Parent = player.Backpack
        clone2.Parent = player.StarterGear
    else
        script.Parent.MouseButton1Click:Destroy()
        --script.Parent.BackgroundColor3 = Color3.New(.5, .5, .5)
    end
end

script.Parent.MouseButton1Click:connect(function() find(player.userId, ID) end)