Is Gemini Sector High-Resolution Nav Map free?

Yes. Gemini Sector High-Resolution Nav Map is released under CC-BY-SA-3.0 and costs nothing. Neither it nor Privateer Gemini Gold has any paid content.

What do I need to run Gemini Sector High-Resolution Nav Map?

Privateer Gemini Gold, which is itself free. Gemini Sector High-Resolution Nav Map is an add-on and will not run standalone.

How do I install Gemini Sector High-Resolution Nav Map?

The PlayBound Launcher installs it in one click from Privateer Gemini Gold's mods list. You can also install it by hand — full steps are on this page.