I believe MyTrack allows you to open a delayed account for free.
They have a limited stock screener, but gaps is one of them.
You can also see how the stock is trading in delayed time.
In the mytrack prog its in "Market pulse".
Nextrend may be worth a look if you what something a little less delayed...10min to 1 min depending on which plan.
You can select which exchanges to include/exclude
"open gap' is one criteria you can can use
Low end plan for the whole program is only $14, no extra fees. http://www.nextrend.com/analysis/features/marketscan.asp
