As law professor Steve Vladek pointed out, “[the] Trump administration decided that… Gordon would’ve had to become the Acting DNI on August 15… and they couldn’t abide that result.” Hence the resignation.
She could have refused. That would have left Trump with two options: If he’d fired her, he might not have had the legal power to appoint an acting replacement. That power applies only when an executive officer “dies, resigns, or is otherwise unable to perform the functions and duties of the office.” It’s unclear if “unable” includes firings. If he hadn’t fired her, she could have served as DNI for a while, maybe saving her resignation for a moment when it would have had more of a rhetorical and political affect, à la James Mattis.
But she did resign, and Nunes is measuring drapes for his new office.