Quote from Charlie Dow:
Question, was this in the cash Market? If it is please tell me what single instrument you use to trade it.
I had the same question.
I don't think that you can use a cash index for a gap study.
I believe (and I could be wrong) that the cash index will open the day using the first traded price or last quoted price of any stock. So if, for example, half of the index doesn't trade in the first minute then the previous day's closing price of those stocks are used to calculate the index.
I believe that the only truly reliable gaps to play are ones which have overnight markets with liquidity supported by a pit market during RTH. Classic would be ES. The overnight market establishes a fair value gap before the pit adds liquidity to the market.
By pit - we can also assume an underlying cash market as well as regular business hours for traders. I say that because I believe that all the pits will be gone within 10 years but the concet of RTH will remain in the form of the restricted trading hours for the underlying cash and business hours of regular day traders.
(Wild assumptions I know.)