Here is the Japanese way to green clean gourds - inside and out.   Chop the top off the gourd, drill small holes in the bottom and submerge   (fully) in water for up to 3 weeks!!!!!!  Yes, I did say 3 weeks.  It will probably STINK - that's the big drawback of this method.  Anyway, after all this time under water, the insides of the gourd are yucky  and sloppy, to say the least, and will all just flop out all over the  ground. I threw some sharp stones inside mine, and shook vigorously, to  fully clean the inside.  Then to the outside, just scrape the green skin off with a blunt butter  knife.  Now, there is a second major drawback to this method, about HALF of your  gourds will rot!!!!!!!!!!!!!!!!!!!  However, for the ones that don't, it's a fabulous way to have clean green gourds.