I have read in Bennett - Trading Volatility the following quote.
As shown above, a long gamma (long volatility) position has to buy shares if they fall, and sell them if they rise. Buying low and selling high earns the investor a profit. Additionally, as a gamma scalper can enter bids and offers away from the current spot, there is no need to cross the spread (as a long gamma position can be delta hedged by sitting on the bid and offer).
However, I am not sure why this is the case. After each move in the underlying, the long gamma investor needs to either buy or sell additional shares to maintain his delta hedge. I.e. if the stock drops he needs to buy more and sell more if the stock rises. Thus, he either needs to sell at the bid or buy at the ask. Why would he be able to sit on both sides?
Furthermore, why would this not be available to a short gamma investor? For me it is simply the same situation, but reversed