Walter, the "pro gap" vs "amateur gap" are just terms from Pristine's methodology. They don't *know* who's causing the gap, but they assume so by the price action.
If you see a gap that breaks out of a trading range, Pristine says that this is most likely professional money that's causing the gap. The reasons they (Pristine) give are the same as you'd find in
VSA--they pros are trying to run the stops of traders who thought the range would resolve in the other direction and simultaneously convince traders who are already in the trade and making money from liquidating.
If you see a gap that extends an already-existing trend, Pristine says that this is most likely the retail traders who are all jumping in late in the game, driven by greed--they don't want to miss the boat. These gaps are often the last gasp of the bulls before the trend reverses, and thus they are called "exhaustion gaps."
Hope I've made this relatively clear.