The straight pushback is always available, and it's not related in any way to the AFCAD. In fact, the main reason for it to exists, is to allow SOME pushback, even if those sceneries the developers forgot to set a pushback direction.
So, regardless of the GSX customization setting or the AFCAD, straight pushback is always available. BOTH means that BOTH Left and Right are available, so they are in addition to the straight pushback.