This is a free application for grating couplers design. It is built on CAMFR framework, which is built in python. To know how GCSim works refer to CAMFR manual at [http://camfr.sourceforge.net/].
Link to download GCSim and its user manual: https://www.dropbox.com/s/vw5w3km8bd3rl11/GCSim_V8.6.1.zip?dl=0