Kroger 4X fuel points on Amazon gift card (possible YMMV)
I bought $100 Amazon (fixed face value) and $150 TRU yesterday and got 700 points. Looks like the amazon gift card earned 4X points. Today I went to another Kroger and bought $200 Amazon gift card (variable face value) and confirmed it. See the receipt for details.
Not sure about other gift cards except TRU (2X points).