Hi,
I'm a big fan of g and use it everyday in may of my projects. Thank you for making and maintaining it.
As far as I can tell, ruby_gntp is incompatible with Growl < 1.3, the consequence is that it's incompatible with Mac OS X < 10.7
Guard had the same issue (guard/guard#154) and I think they choose to autodetect with notification libraries are available, using ruby_gntp if available, or falling back to others (including growl, …).
It'd be great if we could use g >= 1.6 on OS X < 1.7.
Thanks