If bleach wouldn't remove the stain, its just meant to be a part of the gourd's design.........or not!
The best solution is to 'green-scrape' the gourds. That is to actually use a dull edged knife, or other object to scrape the still green skin from the gourds. Then dip them into a mild bleach-water solution and let them complete the drying time. They will also dry faster by green-scraping them.

Jan