I found three issues:

:Gm# is not anymore implemented and replaced by :GX96#

:GX96# does not return a value since threr is may be a typo in OnStepX "Goto.command.cpp
case '6': reply[0] = "EWB"[preferredPierSide - 1]; reply[1] = 0; break; // preferred pier side

:GX90# must effectively be called by
getCommandSingleCharResponse(PortFD, GuideValue, ":GX90#"); //old getCommandString

with these modifications it works fine , everything responds in "real time"