Ghanima is Pre-Born. She awoke to full, adult consciousness before birth, receiving the genetic memories of both her male and female ancestors. This means she's got a LOT of tau awarenesses in her head, basically going all the way back to the dawn of Humanity. Yes, they are genetic memory-imprints of real people. Yes, they're real. No, she's not crazy when she's muttering under her breath to people you can't see. What they know, she knows, be it combat or history.

Being Pre-Born also means she also has the Atreides Curse lingering at the fringes of her mind; Ghanima has the barest bit of prescience. It manifests as instinct and flashes of intuition, or through playing with the Dune tarot. If she thinks something is a bad idea? STOP. She's probably having a pre-cognitive moment.

Last, but far from least, anything a Bene Gesserit can do, Ghanima can do too. This involves using the Voice to control people, simulflow multi-tasking, limited teleportation via the Weirding Way, and Truthsaying (when in-tandum with the tau of Mohiam).