The safer time to sell vol is after it's already dropped, and I know that sounds backwards. Say the gamma data tells you the street got longer gamma on a rally. If that's true, market makers got long, so they should be offering vol down a bit. So check. Did implied vol actually come in? If it did, that's the data being backed up by what the market's doing, and now you can sell vol in that name a lot more safely. If the gamma number went up and the vol didn't budge, I wouldn't lean on the number. The instinct is to sell when vol's in the 70th percentile because it's high. I'd rather watch it drop first and let that confirm the street's long. It's counterintuitive, I know. But in a high gamma environment realised tends to get crushed, so there's usually still plenty of juice left to harvest after the confirmation.