JBH has already pretty much answered the question, so this is just an additional:
The Gemini Program: Physical sciences experiments summary document actually has the same photo as the OP and states:
The crvogenic spectrometer and radiometer erected on the Gemini VII spacecraft
Not to take away from the brilliant research in JBH's great answer, I noticed something about radiometer access hatch in one of Gemini's drawings in the manual.
This then led me to this link:
http://www.collectspace.com/ubb/Forum29/HTML/002005.html
which essentially asks similar questions, about a photo of Gemini VII:
resulting in:
Go4Launch posted:
These experiments, formally known as Celestial Radiometry and Space-Object Radiometry (D004 and D007), also flew on Gemini V. According to "The Gemini Program Physical Sciences Experiments Summary:"
The radiometer and spectrometers were mounted on swingout arms in the Gemini retroadapter section. Doors in the adapter were opened pyrotechnically, and the three sensing units swung through the openings into boresight alignment with the spacecraft optical sight. After the sensing units were erected, the spacecraft was pointed at the desired region for measurement.
This might not be exactly the same package as JBH pointed out there were several flown on different missions, but hope this can help out.
The Gemini Program: Physical sciences experiments summary
https://ntrs.nasa.gov/citations/19720003207
Hires photo on flickr:
https://www.flickr.com/photos/nasa2explore/9492725107
