Help with GUI collisions

Help and Feedback Scripting Support
#1

I’m working on a 2D platformer using GUI objects, and I’ve been trying to get collisions with player and ground just right, but the player always ends up a bit deep in the ground than supposed to.

local player = script.Parent.player

local speedX = 0
local speedY = 0
local GRAVITY = 1

local x = player.Position.X.Offset
local y = player.Position.Y.Offset
local startX = player.Position.X.Scale
local startY = player.Position.Y.Scale
local playerSizeX = player.AbsoluteSize.X
local playerSizeY = player.AbsoluteSize.Y
local lastPos

local function changeVariable(var, by)
	local returnVal = var + by
	return returnVal
end

local function moveX()
	x += speedX
	player.Position = UDim2.new(startX, x, startY, player.Position.Y.Offset)
end

local function moveY()
	y += speedY
	player.Position = UDim2.new(startX, player.Position.X.Offset, startY, y)
end

local function checkForCollision(groundContainer)
	lastPos = y
	moveY()
	for i, v in pairs(groundContainer:GetChildren()) do
		local x1 = v.AbsoluteSize.X
		local y1 = v.AbsoluteSize.Y
		print(v.AbsoluteSize.Y)
		if player.AbsolutePosition.Y - playerSizeY >= v.AbsolutePosition.Y - v.AbsoluteSize.Y then
			y = lastPos
			speedY = 0
			moveY()
		end
	end
end

local function movement()
	speedY = changeVariable(speedY, GRAVITY)
	checkForCollision(script.Parent.ground)
	--player.Position = UDim2.new(startX, x, startY, y)
end

while game:IsLoaded() do
	movement()
	wait()
end