I made a new document, size 1024x1024. I brought in a flat plane, went into edit mode, hit F to frame it. But when I export the image it comes out like this?
What am I doing wrong? I want it to frame only the plane, which it should since it is a perfect square.
The mesh is framed to the visible portion of the canvas, so the probable explanation is that part of the canvas is hidden. This happens if the canvas is larger than the available screen space. You can use the Document>Zoom to reduce the scale of the canvas so you can see it all. This won’t affect the size of the exported image.
If you set document range, rate and center to 0 the gradient will go away. 
edit: I got it to work by first zooming out and then framing the document. Though it still exports with a 2-pixel border on the top and bottom… whatever, Zbrush is strange some times. Ignore the attached pic.
The two pixel border is a different issue. I’m afraid the best way to deal with that is to trim it off in Photoshop. Obviously you’d have to use a larger canvas to accommodate it.