In LRM 2.2, you're supposed to be able to get it by
$simparam("gmin")

However, I don't believe any simulator supports this yet, so you probably have to set up a parameter in your module
 parameter real gmin = 1e-12 from (0:1);
and then pass the value into the module from the instance line in the netlist.  Presumably, you can access the simulator's gmin from the netlist.