Looking at Gemini´s spacesuit G-4C I always wonder how did they make the neck joint able to swivel, allowing the user to look at both directions? Wouldnt be easier to make a slightly bigger helmet (i.e. SK-1 for Vostok) with a large faceplate visor?
Let's find a bigger version of the image that you attach and blow it up:
As you can see, they used bearing with an arc rail to hold the umbilical in place. The outer ring of the bearing is attached to the collar of the suit, and the helmet slides into a locked position and attaches to the inner ring of the bearing. Similar bearings are still used on newer EVA suits in hips and gloves, and some newer designs to support future human exploration of Mars use even more of them at various angles (two in pairs at a canted angle to each other) to enable more natural joint movement.
See image gallery at the source of above image I crop (basically I found it by drag-dropping your image into Google Image Search's search bar and cropped from the highest resolution it found), and there's some more information regarding bearings on NASA's Learn About Spacesuits page.
I don't know the exact details of the Gemini suit, but here are some contemporaneous patents that describe similar joints. The different parts are in red and blue, bearings in green and o-rings yellow.
Omni-directional high altitude helmet, US3030626A by Leonard F Shepard, filed 1958:
It's basically a large ball bearing with an o-ring, both in such an arrangement that the internal pressure makes them tighter.
High altitude helmet, US3293659A, again by by Leonard F Shepard of International Latex Corp, filed 1964:
This patent most resembles the Gemini helmet. This apparently only has ball bearings and a tight fit...
Finally, Hard space suit, US3405406A by Hubert C Vykukal of NASA, filed in 1966:
Again, a bearing (oriented differently) and an o-ring, made "of a plastic material such as nylon or Delrin".