What does the "P" in "SP" and "GP" stand for? I take it you're talking about special relativity and general relativity? In special relativity spacetime is flat everywhere, so a frame doesn't have to be local for the speed of light to be c, it just has to be "inertial", meaning that any observer at a fixed position coordinate in a given frame is not physically accelerating (they feel no G-forces). An inertial frame is usually thought of as being constructed out of a grid of rigid rulers and clocks which are all at rest with respect to one another and moving inertially. In general relativity spacetime is curved so long rulers can't really remain "rigid", but if you zoom in on a small region of spacetime curvature effects (tidal forces) can become negligible, so this is where you can have a "locally inertial" frame, in a small region of spacetime where tidal forces are not detectable. In such a region, a locally inertial frame can be thought of as being constructed out of rulers and clocks which have no non-gravitational forces acting on them, meaning they are in freefall. For more on this, please read http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html .