When the OP asked, I think at a certain level of spending--say 20K or above--Amex would care, but not now with the cap.
At $6K per year, Amex couldn't care less how you spend do it. As always, better to mix in some groceries.
The only points where Amex seems to care how it's accumulated is SPG, and to a lesser extent, MR points. In this case, they care about points dilution and the watering down of the program, especially with SPG points.
So, when doing this for SPG points, tread carefully. Cash on the other hand, Amex doesn't care about.
Keep in mind the grocery 6% cap so you can only get $360 if you buy all gift cards - $75 AF. Don't think it would be a problem it's not that much cash back.
I've bought Amazon cards and cell phone cards. I've seen prepaid Amex and Visa cards, but it never crossed my mind to buy those... But I suppose that would be a good way to max out your 6K limit. I suppose one issue with those cards would be plannig your purchases to fit the leftover balance you have on them and you wouldn't be able to get rid of the few dollars left on there or at least not easily, it would take some extra planning, because you can't refill them or at least that's my understanding. Say you have an Amex gift card with $15 left, do merchants allow you to use up that card and then pay the rest with a regular CC? Idk, seems complicated, but if anyone has been doing that pls let us know how that works.