Question What is this plastic around gpu chip?

It serves the same function as the protection shim for the old Socket A processors did--as the GPU is a bare die and the heatsink is springloaded, it prevents cracking of the corners of the die if the heatsink is ever impacted and tilts, or is tightened unevenly.

This would not be required with a more solidly mounted heatsink assembly, or with an Integrated Heatspreader (IHS) over the die like all modern CPUs use