In 3D Euclidean space, the position of a point is uniquely determined by its distance from three other (distinct, non-colinear) points (mostly; in some cases there may be two solutions). Trilateration is the process of finding a point from three of these distance/point pairs - strictly speaking, this is "true range" trilateration (or, more generally, multilateration), and the more widely used pseudo-range trilateration (multilateration) uses four distances instead. GPS, in ComputerCraft and otherwise, is based on this process, though ComputerCraft GPS is free of almost all noise and sources of error and thus uses much simpler algorithms and can achieve near-perfect accuracy easily.
ComputerCraft GPS uses at least four GPS servers which broadcast their position when requested (an implementation detail which has led to many interesting information leaks itself, requiring development of passive GPS), with clients measuring distance and performing the trilateration computations. As all ComputerCraft wireless broadcasts provide exact distance by default, without any special design or time sync being necessary, it is also possible to use identical maths and an array of receivers to locate the sender of any wireless message. This capability has been extensively used for monitoring, especially since Opus OS broadcasts periodic network pings.