The most important thing you can do is help her relax about it as much as possible. You might want to investigate stress-management techniques for children and allow her to practice relaxing so that she can use these techniques in a testing situation. The book Stress Can Really Get on Your Nerves by Trevor Romain and Elizabeth Verdick is filled with good ideas and strategies that are appropriate for children your daughter's age.
Above all, you need to stay calm about the test, because she will pick up on any worries or anxieties you have. This might inadvertently make her think that your feelings about her are dependent on whether or not she achieves. Before and after the test, let her know that you love and value her no matter how she does on the test, or whether or not she gets into the gifted program. Try to use this opportunity to teach her skills of handling stress and disappointment by telling her how you have coped with difficult circumstances in your life. Those are the kinds of skills that she'll be able to use her entire life in a wide variety of situations.