Let her know the field isn't "her" space, and she's not free to go the minute you walk through that gate. My yearling went through a stage where he'd give a "buck-kick" in my direction when I turned him out a couple times. I made it a habit of taking him through the gate, and then backed him high speed halfway across the field, and made him do some pivots, picked up his feet etc general stuff, then he had to stand quietly for a full minute. I also did the same thing MN Tigerstripes mentioned and loped the leadrope around his neck before unclipping the lead so he didn't associate that with immediate freedom. He learned he's not free to go until I walk away, and the problem never happened again.