The GDO really only does 2 things. First, it will move you to the end of the list of available Pilots eligible to be junior assigned on a day off. Second, if you do get junior assigned, you can decline or accept and get an additional 6.5 tfp on top of whatever the flying credit pays.

It will not prevent you from being junior assigned, and it does absolutely nothing if you place it on a day you already have flying or reserve.