Why go thru the script-fu thing when you can a call GMIC directly from Python?
(changed your "input" parameter to "1" because "0" does not show any effect).
If you want to pursue with the intermediate script, check that it is registered correctly. If it is, it should show up in the PDB browser in the Python console, and be part of the PDB so you can call it directly: pdb.script_fu_engrave(image...)
Code:
pdb.plug_in_gmic_qt(image,image.active_layer,1, 0, "fx_engrave 0.04,40,0,2,45,-1,1,0,10,1,0,0,0,1,0,50,50")(changed your "input" parameter to "1" because "0" does not show any effect).
If you want to pursue with the intermediate script, check that it is registered correctly. If it is, it should show up in the PDB browser in the Python console, and be part of the PDB so you can call it directly: pdb.script_fu_engrave(image...)