How do I tell a script to find a model within a certain range of it?

well, it depends, if you want to know when a specific part is in range, than this should work;

local function in_Range(part1,part2,range)
	if (part1.Position - part2.Position).magnitude <= range then
		return true
		-- alternatively
		-- return part2
	end
	return false
	-- alternatively
	-- return nil
end

debounce = false

while wait(0.5) do

	if in_Range(game.Workspace.gate,game.Workspace.car,30) == true then
		if debounce == false then debounce = true
			game.Workspace.gate.CanCollide = false
			wait(3)
			game.Workspace.gate.CanCollide = true
		debounce = false end
	end

end

or, if you want to see if any part in a model is in range, then replace the last part with this;

while wait(0.5) do

	if debounce == false then debounce = true
	
		local carModel = game.Workspace.carModel:GetChildren()
		
		for i = 1, #carModel do
			if in_Range(game.Workspace.gate,carModel[i],30) == false then
				debounce = false
				break
			end
		end
		
		if debounce == true then
			game.Workspace.gate.CanCollide = false
			wait(3)
			game.Workspace.gate.CanCollide = true
		end
	debounce = false end

end

or if you want to know when any part at all is in range, I hate you.