Why would the Jumpships plummet into the stars due to gravity. I thought Jumpships jumped to locations with no gravity?
There's no such thing as "no gravity points."
The standard zenith/nadir points, as shown in DS&JS, BattleSpace, Explorer Corps, AT2R, and Strategic Operations, are stationary points over the poles of a star. They are points so far from a star where gravity is low enough for a hyperspace field to form safely. You can also find similar conditions at L1 Lagrange points.
The standard zenith/nadir points of Earth's solar system are only 10 astronomical units from Sol's north and south poles - only as far from the sun as Saturn. Trust me, the sun's gravity is doing a fine job of keeping Saturn in orbit, as well as planets further away like Uranus (20AU) and Neptune (40AU) and the even more distant objects.
Zenith/nadir points are actually pretty deep in a star system.
Unlike an orbiting planet, the zenith/nadir points are not orbiting. They're stationary. There's no centripetal force to resist gravity. When you turn off the stationkeeping drive of a JumpShip or recharge station, the ship begins falling toward the star.