traders try to trade a gap..however its widely recognised that the gap doesnt have to be filled - it can be 'hit' before trading recommences going upwards. Volume being traded has a lot to do with gaps up.

If you check the GPR chart it was trading for some while before the big break out with a very flat daily trading spread. That would make it very difficult for day traders to jump in and out,and so probably created a buy and hold interest over that period. The buy that eventually created the big spike wasnt 'that' big.

The red/green line is the technical support line that's been created by previous trading. Support for the stock is created by previously used buying levels, and potential selling points are created at levels when former investors have been left at higher price points , and may choose to bail out.

A strong buying support line exists as its now being retested at a slightly higher level. Despite the gap, GPR appears to be trading up and in line with other silver Canadian juniors, so appears to have filled it. Has also bounced off a fibonacci retracement line.