nsclient++ out of the box won't do that (unless, of course, you can think of a WMI query to get that information)..
If, however, you write a script/program which responds in the nagios fashion (see nagiosplugins.org/ for development guidelines) then you can instruct nsc++ to run that command.
All nagios cares about really, is state code.. Is the service/host unknown, up, down, unreachable etc.?
Ie. You could write a command that parses the output of netsh or ipconfig to determine current gateway settings and compares them to a known good value.
... there might even be something already to do this - good place to start is nagiosexchange.org/