Got gas at a location never bought from before, contained 10% ethanol. The catcode P0420 popped a day later. O6 Pontiac Grand Prix 3.8L V6?

And, do you have a question?  The fuel you put in would not cause that error, a leak failure in your exhaust would.  Get if fixed.

10% ethanol by itself would not cause this code. This is cat below efficiency. The only times I have seen this code is when there is a leak in the exhaust system. This could be a bad gasket or an cracked weld. There appears to be a "flex joint" in the area of the cat and that could have blown out (failed).