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 crvogeniccryogenic spectrometer and radiometer erected on the Gemini VII spacecraft
There are photos of the actual experiments in the document as well, as well as showing their locations.
Thus: Top left in the OP photo is the Proton electron spectrometer and bottom right is the radiometer.
...
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.
The Gemini Program: Physical sciences experiments summary
https://ntrs.nasa.gov/citations/19720003207
Hires photo on flickr:
https://www.flickr.com/photos/nasa2explore/9492725107
pictures overlaid:
