To create a field function based on Grid Flux that has the same units as velocity, create a new field function called, for example, Grid Flux Velocity. Set its dimensions to Length = 1 and Time = -1, and set its type to Vector. Set its definition to:
$$GridFlux * $$Area / $$Area.mag2()
In this expression, $$GridFlux / $$Area.mag() is the magnitude of the velocity, and $$Area / $$Area.mag() is the direction of the flux, normal to the face area.