An alternate-history "historical" add-on.
Instead of, or as a backup to, Apollo, Gemini is further developed to help win the race to the moon.
In this add-on, a Centaur rocket stage is outfitted with an Agena-style docking collar and launched into orbit.
You will launch into orbit in your stock Gemini (not included), rendezvous and dock, and command the Centaur to send you on a free-return trajectory.
Your ride into space history awaits.
(Requires Gemini-Titan 4.11 or later. Earth_1962 add-on also highly recommended for maximum enjoyment).