No to the GPS, definitely not a necessity. Still I always have my phone which is a gps and do use it now and then on tour. It just is not a necessity. In fact on the Trans America I mailed my GPS home early on.

So, what do you actually need. The answer is not much. A bike is for sure a necessity otherwise it isn't bike touring, but otherwise there isn't much that is an absolute necessity. A lot of things are highly advisable, but there aren't many that someone hasn't successfully done without on a tour.

After a quick read through the things listed in this thread as necessities, I notice that I have at one time or another toured without almost all of the items listed and some I have never taken along.

So what are the actual necessities? A bike, adequate clothing, some means of keeping warm, some means of staying fed and hydrated, and some means of sheltering oneself.

There are other highly advised items, like enough tools and spares to repair and maintain the bike.