The phase observable measured by an atomic gravimeter built up on stimulated Raman transitions is discussed in a fully relativistic context. It is written in terms of laser phases which are invariant under relativistic gauge transformations. The dephasing is the sum of light and atomic contributions which are connected to one another through their interplay with conservation laws at the interaction vertices. In the case of a closed geometry, a compact form of the dephasing is written in terms of a Legendre transform of the laser phases. These general expressions are illustrated by discussing two techniques used for compensating the Doppler shift, one corresponding to chirped frequencies and the other one to ramped variations.