Assuming her motivation in the film is similar to that seen in the official novelisation, the simplest answer is that she sees the Gunslinger as a powerful alpha male figure, one that inspires both elements of primal fear and an overwhelming feeling of masculine sexual dominance.
Tracy drove with carefree abandon, her eyes slitted almost shut against the windstream, her hair whipping about her ears. Chuck wondered for a moment why they were not seeing Tracy’s dream directly from her point of view. Are we all conditioned by movies and television to see outside ourselves? he wondered. But a sudden movement from Tracy brought his attention back quickly.
She had turned to look to the side of the road, and after a blurred transition he saw what she was seeing/dreaming.
A black-clad gunslinger with vivid, rather unhuman eyes was riding a beautiful horse in dreamy slow motion by the side of the roadway, pacing the swift sports car easily. He stared at Tracy with a fixed, insolent audacity as he galloped easily along.
At first he's simply a threatening figure (which gets her adrenaline pumping) but then, with the help of the fantasy technicians, he uses force to save her from being assaulted by a group of men. Clearly she finds this scenario both distressing (on intellectual level) but then confusingly erotic.
Chuck saw Tracy on the ground before the house, surrounded by the seven men in scarlet hoods. Her hands were bound. The men began to close in around her. They held large silver hypodermics in their hands, much as a knife is held.
Chuck’s mouth felt dry.
“Approaching the upset point,” the technician murmured. “Switch to manual, Cee Em, please.”
The black-clad gunslinger rode up to the scarlet house, dismounted in a smooth, practiced, effortless move and started toward the grouping of red-cloaked figures surrounding Tracy.
The men in scarlet turned away from the recumbent figure of Tracy, reacting to the oncoming gunfighter. They ran at him, their silvery hypodermic needles raised high as weapons. But the gunslinger drew as he moved, his gun firing, the sound of the explosions booming and distant, echoing and fading.
He finally asserts his primacy by putting her onto the bed and making passionate love to her.
The mist swirled and Chuck now saw Tracy on the floor of a white bedroom. Mist heavily obscured the outer edges of the image, but the gunslinger strode in and pulled the wide-eyed video reporter to her feet. A hand on her waist, his other hand unraveled the knot of the scarlet cord that bound her wrists. Throwing the cord from him contemptuously, he pulled Tracy to him and kissed her hard. But she broke away, backing toward the white bed.
He wheeled toward her, his dark figure blurring into merely an abstraction of motion, a dark stream across the Mind Flowing screen.
The filmmakers are trying to make it clear that on a subconscious level she's been powerfully influenced by the footage she saw earlier in the film. She has mixed feelings of both repulsion and attraction to the Gunslinger and in her dreams they tip the balance more toward the latter through mind-controlling techniques.