My opinion is SIS (sometimes called ESD Emergency Shutdown System).
FGS send the shutdown command to SIS via hardwired.
If you have FGS cause and effect diagram, you find the detail.
Initiation in case of fire or gas detection is always by F&G detectors through F&G Panel or inbuilt with SIS system depends on design. however, F&G is third layer of mitigation after failure of SIS.
But effect of any ESD is always from SIS if I am not mistaken.