From Investopedia:

“Breakaway gaps don’t tend to fill, or at least not for a long time. Instead, the price tends to run in the same direction as the breakway for some time after. The times when breakaway gaps do fill, the breakout direction usually prevails. The price may move back to where the gap started, fooling traders into thinking the gap was a false breakout, but then the price usually keeps moving in the breakaway direction.”

I don’t know whether a chart expert would verify yesterday’s GDX gap as a “breakaway gap”, but volume sure was pretty heavy.