There is no grease. As pjc50 has pointed out, an integrated GPU is part of the die along with the CPU.
The two packages are different approaches to BGA packaging. This document gives examples of your packages on page 14-4. The left package is an HL-PBGA die-up package, and the "shiny mirror thing" is actually the bottom of the die. The other package is an H-PBGA die-down package - if you were to dismount the package you'd see a well in the underside with the chip inside.
In both cases, thermal contact with the heat sink is made by thermal conductive pads
rather than thermal grease. Thermal pads are not quite as good (thermally) as a good grease, but they can be much thicker, and they have much more tolerance for differences in separation between the IC and heat sink. This allows adequate performance at much lower cost than would be needed to handle multiple chips, each with a different surface height. And you can see that two different pads (with different nominal thicknesses) have been used for the two chips. Note also that the left-hand pad has compressed around the die, evidence of good mechanical contact and pressure.