gamma measures how much the delta changes for a given change in stock price.
the point of a delta hedge is to create a position that is insensitive to changes in the asset price. You create this position using the delta. if the delta changes, you need to rebalance, the more the delta changes, the greater the extent of the rebalancing. hence when gamma is large, delta is volatile and more rebalancing is necessary making it a costly and not such an effective strategy.
remember an effective hedge value would ideally exactly offset the movement in the asset. won’t happen if you dont have the necessary stock/option ratio.