From the log it looks like it could be a syntax error, but without seeing the netlist it's difficult to tell what the actual issue is.
The GNT github page also mentions you should use "Any reasonably recent MMSIM version.", which I'm not sure MMSIM 15.1 qualifies as since it's ~6 years old, so perhaps some constructs not supported by legacy versions are being used.
EDIT: Just noticed that the getting started document is dated 2015 so I guess it's unlikely to be an issue with using an "old" version of Spectre.