If there is no trading taking place in the futures, then how can there be a gap in the futures? I have seen it very rarely. Once was a couple of weeks ago, but it was not enough to break any bank for a futures trader. There is no "gap" per se in futures.
Of course there is GAP in future trading.When it hits Sunday evening 6:00pm, whatever news happened in the weekend, the index future (ES etc) price can be dramatically up or down from the close on previous Friday.