GfsSourceFlux is used to add a flux distributed over a given area (in 2D) or volume (in 3D) to a variable.
The syntax in parameter files is
[ GfsSourceControl ] [ GfsFunction ]
The first GfsFunction gives the value of the flux (with dimension [Variable] x L2 x T-1 in 2D and [Variable] x L3 x T-1 in 3D). The second GfsFunction gives the area (in 2D) or volume (in 3D) fraction over which the flux will be distributed.