You can not change maximum iteration number of gradient computation with the GUI (iterative solver /= gradient).
Do you have an idea of your mesh quality?
You can try to use a centered scheme for convection (or SOLU) with least square gradients over partial extended neighborhood computation.
There are validation reports for production versions of Code_Saturne. But, at the moment, due to validation test-cases based on industrial and nuclear applications the validation report is internal to EDF.
You may use the subroutine uskpdc (uskpdc.f90) to impose head losses. You also can use ustvnv (cs_user_source_terms.f90) as said Brian to impose momentum source terms.