Gap

The area between the two poles of a dipole magnet, where the magnetic field is concentrated. Also used to refer to the aperture.

Gauss

The cgs unit of magnetic flux density. Officially declared to be outdated, it is still popular for low fields. Equal to 10-4 Tesla.

Gaussmeter

See magnetometer.

Gradient

In this context, gradient refers to the spatial variation of the magnetic field.

Gyromagnetic ratio

In NMR or ESR, the ratio of the resonant frequency to magnetic flux density. Varies from one nucleus to the other; for hydrogen (protons) it is about 42.5 MHz/T. For the electron, it is 28 GHz/T.