Suggestion is:
in cases where there's a narrow space, instead of using gap fill, extrude the adjacent tracks more widely
background: e3d nozzles go up to around 2.5 * the nozzle size in terms of extrusion width possibility (the flat zone of the tip). That leaves a lot of room for "filling in" without resorting to gap fill which, if applied layer after layer, tends to cause clumping, not to mention being a very staccato sort of motion and not particularly fast.