Numerous websites such as this Time article or this Nerdwallet page say you can buy gift cards at grocery store to get the higher category rate when applicable.
You can purchase gift cards at supermarkets for Amazon.com, Starbucks and many other chains, giving you an effective 6% cash back at those stores.
Source: Best Cash Back Credit Cards of 2016
I just talked to American Express online chat and the rep I talked to said this is not and has not ever been the case. I said,
"So if I go into the grocery store and buy $30 in groceries and then get a $20 starbucks card, I don't get the 6% on the starbucks?"
and the rep said no that portion would not be included.
Could someone tell me if this rep is correct, I find it a little concerning that there are many reputable websites and publications that all say otherwise.