I'm super confused about this, I went to look at some land available for sale, and its all govt property, and a military base right below. Is the government selling land? Is it an active base? Open to the public?

Government don't sell land. When you look for a land near FT hunter Leggett, look for sign it should say US government property. Also base is active but you can't just come in and see it. You have to be affiliated to the service or to the government. But you can buy hunting pass and follow the policy for hunting if you like the wilderness and want to hunt

There's a bit of land just outside of the base, some of it across the street it seems, for sale... Fort Hunter Liggett is not selling off land

The government is not selling its land. You might be looking at property along the skirts of Fort Hunter Liggett.