People use "GPS" the way they use "hoover" or "biro", as a catch-all for a category. But locating a phone actually involves at least two very different techniques, and they answer to different masters.
GPS is the satellite method. Your phone listens to timing signals from orbit and works out its own position, as we covered in how phone GPS actually works. It is precise, often to a few metres in open sky, and it is beautifully private, because nothing is transmitted from your phone to make it happen.
GSM triangulation is the cell-tower method. Your phone is always in a quiet conversation with nearby masts, and the network can estimate your position from which towers you are talking to and how strong each signal is. In a dense city with many towers this can be reasonably good. Out in the countryside, where one lonely mast covers miles, it can place you somewhere vaguely correct and enormously large.
The crucial difference is direction. GPS is something your phone computes for itself. Tower-based location is something the network can estimate, which is why it sits behind legal safeguards and is not simply available to curious members of the public typing a number into a website. That last point matters, because a great deal of internet folklore assumes otherwise.
In practice modern phones blend everything, GPS, towers, wifi, motion sensors, into one confident-looking dot, and hide the messy uncertainty behind a tidy circle. Our tracking page shows how those pieces fit together.
So when a film has a hacker "triangulate" a target in seconds from a laptop, enjoy it as fiction. And when a site like ours draws a dramatic sweeping trace across a map, enjoy that too, as the affectionate joke it is. The technology is real. The idea that you can quietly aim it at a specific person from your sofa is mostly not.