Help about SNAP Graph Processing Framework (GPF) can be accessed from SNAP-Desktop. From the sources, you can still acess help by downloading SNAP-Desktop and opening html files in snap-gpf-ui/src/main/resources/org/esa/snap/core/gpf/docs/gpf, like GpfOverview.html or GraphProcessingTool.html
For your example, you should open the image (org_esa_snap_gpf_operators_standard_ReadOp.html), subset it (org_esa_snap_gpf_operators_standard_SubsetOp.html) and then write the product to GeoTiff (org_esa_snap_gpf_operators_standard_WriteOp.html)