The Gemini program allowed astronauts to expand their skills into areas that had not been looked at before.
The Gemini missions were the first missions that allowed astronauts to change their orbit during flight. This was the technology that would make it possible for the Apollo missions to go to the moon.
The Gemini program allowed its astronauts to participate in spacewalks. They were able to test the limits of what was possible and capture many pictures of what they observed. The astronauts' experiences made NASA revise the way they trained for spacewalks and showed that more work on the ground was needed to perfect the ability to work in space.
Gemini VIII was the first mission to connect two spacecrafts while in space. This skill would be very important for the missions to the moon. Gemini X would combine linking two spacecrafts with retrieving a package with experiments from a second spacecraft (seen above).