I'm not exactly certain what this is called or what the precise purpose is, but
it is an assembly of extra shingles which are attached to the hatch via a hinge
and "float" over the shingles of the crew and equipment section's exterior.
The extra shingles appear to be attached to the exterior of the crew and
equipment section as well as attached to the hatch, and appear to slide on pins. Gemini 8 is rotated so that the pilot's hatch is higher off
the ground, so it was more difficult to photograph than, say, Gemini 12.
Unlike Gemini 12,
Gemini 8 does not have these shingles on the other side of the spacecraft. Unfortunately, I
did not think to inspect that side for any associated hardware.
Picture 5 of 6.