What color and texture is the paper of the scanned page? Do you wish to retain that color/texture?
If it’s white and there is no texture, that you want, then Silkrooster’s #2 is very good, and you can run Threshold on that area if need be. You might feather the Mask a bit on the "inside" edge.
If you want color and texture, AND have enough of a margin on the outside of the page to work with, you can create a Selection (Rectangular Marquee, Lasso, Pen Tool Path->Selection, etc.) of that clear margin, Ctrl-c (Copy), then Ctrl-v (Paste) to create a New Layer. Move it to the offending margin, and add a Layer Mask to hide any sharp deliniations, faults, etc.. If you need more details, should any of these prove useful, please ask.
If you have "ghost text" in addition to the gutter, you’ll need to do a Select/Color Range or magic wand selection of a light gray color value and feather it with a very tight pixel radius in addition to the gutter before filling with white.
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