need help fixing GUI ?

There are some structural things you should change to clean things up.

In general, it is bad to make functions change global variables. They should instead return results. E.g., instead of defining local pack at the top and making getMyPack change pack, it should just return that value:

function getMyPack()
    return game.Lighting:FindFirstChild(ply.Name .. "sPack")
end


....

local pack = getMyPack()

That said, you later use the expression game.Lightin .... .. "sPack"), even though you have both a function to get you the value, and a variable!

Once again, we can eliminate the global variables x and y. They are only effectively changed by the function New, so they should be local to that function.

Your loops at the bottom are just plain strange. I'm not sure why you have three loops.

If you want to add a new Configuration for each item in the lists, you should do something like this:

(You should also have the list definitions probably be near this rather than all the way at the top to avoid temptation for other places to touch them)

local nameList = {
    "1", "2", "3"
}
local valueList = {
    "1", "2", "3"
}
for i = 1, #nameList do
    local n = gui.Configuration.Value:Clone()
    n.Name = nameList[i]
    n.Value = valueList[i]
    n.Parent = pack
end

Another minor detail is that you should probably :Clone() the Button you get at the beginning; you should also be using :Destroy() instead of :Remove().

Your variable names all over the place aren't good. list1 doesn't describe what it is; a name like nameList would be much better.

New is similarly bad. UpdateSlots is much more accurate and explanatory.

Note that you don't need to make a new function when you're connecting to the ChildAdded and ChildRemoved events -- it will just call UpdateSlots if you just give it UpdateSlots.

Overall it would look something like this -- this is untested, you should make sure you can get here yourself:

local gui = script.Parent
local Button = script.Parent.Configuration.a1:Clone()
local slots = gui.Frame.ScrollingFrame.FrameH
local ply = gui.Parent.Parent

function getMyPack()
    return game.Lighting:FindFirstChild(ply.Name .. "sPack")
end

function removeOldOutOfTheWay(objects)
    for _, child in pairs(objects) do
        if child.Name == "a1" then
            child:Destroy()
        end
    end
end

function UpdateSlots()
    removeOldOutOfTheWay(slots:getChildren())
    local x, y = 0.3, 0.3
    local pack = getMyPack()
    local lo = pack:getChildren()
    for _, child in pairs(lo) do
        neww = Button:Clone()
        neww.Position = UDim2.new(x, 0, y, 0)
        neww.TextLabel.Text = child.Name
        neww.Parent = slots
        x = x + 0.15
        if x > 0.7 then
            y = y + 0.015
            x = 0.3
        end
    end
end

local pack = getMyPack()
pack.ChildAdded:connect( UpdateSlots )
pack.ChildRemoved:connect( UpdateSlots )
UpdateSlots()

local nameList = {
    "1", "2", "3"
}
local valueList = {
    "1", "2", "3"
}

for i = 1, #nameList do
    local n = gui.Configuration.Value:Clone()
    n.Name = nameList[i]
    n.Value = valueList[i]
    n.Parent = pack
end