Why would an attack be planned with the waxing gibbous moon?

A "waxing gibbous . The Italians would like to be able to see where they are going, & whom they are sooting at. As invaders have learned the hard way for centuries, to invade this mountainous region one must take advantage of everything available to them.

- Richard V. Horrell WW 2 Connections

In addition to the above (maximal light conditions while still benefitting from the cover of darkness), a waxing gibbous moon

The reason this is important is for amphibious landings. Very high tides tend to flood beach defenses (which are usually designed for as far safer to use and more likely to reach the beach intact.

One note: modern low-light and infrared technologywi now generally chose to attack during phases close to the new moon ) and in bad(cloudy, ra can have such vision-enhancing equipment) AVOID waxing gibbous moon phases.