Why Remove Image Gap is greyed out on justified layouts

On a justified layout, Remove Image Gap (right-click a photo) and Remove Gap at Book Gutter are unavailable, because each photo's space is calculated from its aspect ratio and the project's image gap together. To change the spacing, change the image gap in Settings - justified layouts re-fit to the new gap exactly. To use those controls, switch the spread to a built-in template, or go into the Template Editor (E) and make any change to the layout. The moment you edit it, the justified layout becomes your own custom layout and the gap controls work again. Opening and closing the editor without changing anything leaves it justified.