I would fall into the camp of not necessary but definitely recommended. A GPS on the handlebars and a large scale map in your tank bag are the best of both worlds.

Traveling between cities can be accomplished pretty easily as others have said with a good sense of direction, awareness of signs, and an affable nature to chat with people to ask which way when you are unsure.

Within cities, it can be a real challenge at times without a GPS. The street signs in some places are hard to find until you figure out where to look and if your Spanish isn't great, the directions can be hard to follow. Just think how bad some directions are even when given in english in your home country. "Turn right at the big tree". Now try to comprehend this all in Spanish, given at a rapid pace.

I was just reminded of this over the holidays in Colombia as I was looking for the unmarked turnoff to a small fishing village. I had to overshoot a few times until finally narrowing to a single dusty street. This is all good fun, but sometimes on a hot day with an overheating bike, you are just happy to get to your destination before your passenger completely dehydrates.

Another example where I missed the GPS, (even though I tend to want to rely on maps and to orient myself through landmarks) is when heading in and out of cities and even returning back to your accomodation. If you stay in one place for a couple of days, you'll figure it all out but if you are moving from city to city pretty quick, riding out to get something and to come back can lead to a few wrong turns and some extra time in the hot sun until you make it back. With your GPS, you just use the track back function and you'll get back rigt away. With one way streets and busy traffic at certain times, a five minute trip can turn into a 30 minute meander pretty quick.

So in short, for a first time rider to Central or South America I would recommend to take a GPS, especially if your Spanish isn't strong. But as others have commented, orienteering and map skills are great to have so have no fear in keeping the GPS off unless really needed to hone these skills and to open up your world to random adventures. That way, you'll have the best of both worlds.