In another thread someone suggested rotating the part 45 degrees to get solid infill to be laid down straight across flat surfaces for what some might consider a better looking finished product. I decided to give this a try, but when I slice, the solid infill stops short of the edge on both sides of the infilled area.
I've reduced the extrusion size from 0.6mm * 120% infill size, to 0.6 * 100% size, to 0.5 * 100% size, to 0.40 * 100% size and at this point finally it fills in the space. I think this means that there's a 0.4mm gap on both sides. However, because I have a 0.5mm nozzle, I'd never be able to achieve this extrusion width. Even the infill algorithm could benefit from some sort of variable extrusion width gap fill, although in this case I believe the infill would have to be aligned to one side or the other first rather than centered in the area to fill.
Given that the infill line is 0.6mm*1.2 = 0.72mm and there's a 0.4mm gap, that 1.12mm. If the algorithm could split the difference and make two lines of 0.56mm, one occupying the space of the current first/last infill line, and a one new one, then it would be filled completely.
The other option (that I went with to fix this specific instance) was to up my Outline Overlap until the space filled, but that won't work for shell gap fill.