But assume that the chemical warfare was on the 7th or 8th of June 1944. The troops were still building up. With enough gas there would be more casualties. The loses would have an effect on the possibility of a next invasion.
1. In order to mount a massive attack on June 7 or 8, the Germans need to understand, on June 6, this is not a diversionary move. Given that in OTL they didn't…
2. Assuming they do, tell us about the delivery means. Remember, the stuff is in the Pas de Calais. Bring it up front and distribute it to the platforms tasked with using it. You'll see it will be a tad difficult.
3. The Allied troops landing in Normandy were exceptionally well equipped against chemical attacks. There would be losses, certainly, mainly due to malfunctioning, torn, misused equipment, and anybody WIA for other causes would almost certainly become a gas KIA. But it wouldn't be a massacre, even with sarin. Additionally, the delivery means that you have to list won't be deliverying ordinary HE, which reduces the other causes of casualties.
4. And I don't think those casualties would affect further landings. Another one was already slated to go, Dragoon/Anvil, and it had other units already set aside for it. The Allies would learn about the events in Normandy and would strengthen their countermeasures; they would, by the time of Anvil, have carried out extensive gas attacks against German cities. The Allies would of course use gases in turn, in preparation and during the landing.