GPS provides the correct time anywhere on the planet, it's a universal (as far as earth goes) clock, accurate to 10 nanoseconds (3m). The different reference planes that opposite sides of the planet (at the equator) gives you a precision of femtoseconds (micrometers at light speed) so makes no difference there.

The quality of GPS receiver of course is important, it may reduce your precision to to 100ns or even 1000ns, but it will give you the right time. Someone equidistant to two GPS synced clocks will see them both at the same time (within a microsecond)