I once stood at the middle of Uyuni salt flat, which is a 10,000 km2 flat surface 3 km above sea level. It was daytime and I had just replaced the batteries for my Garmin hand GPS (this was in 2011). I hold it still for a few minutes and then check the screen. It has a message that reads more or less like this: "Satellite signal is too weak, please go to open space." I look around, see the salt flat extending dozens of kilometers in every direction. I look up, not a single cloud in the sky. I reply to the device: "Bitch, we are at the openest space in the world!"
A few years later, attending to a workshop in INPE, I am told that South America is the only place in the world that has what could be translated as "ionic atmospheric poket", which are atmospheric phenomenon that jams electromagnetic signals from GPS. Googling it today, I could not find that particular name the lecturer used, but I found about the South Atlantic Anomaly, and this article about its role creating above mentioned ionic pockets.
Having lived there most of my life, I can say GPS signal at South America is much less reliable than in the rest of the world, but it is nothing like a complete jamming of all signals, and my particular problem at Uyuni resolved itself after a few minutes.
That said, its is totally conceivable a planet where such atmospheric anomalies, fueled by the interaction between planetary magnetic field and spatial radiation, would be able to completely block GPS signal over some parts of the planet.