When a gourd is thoroughly dry, its seeds will rattle when the gourd is shaken. At that point, scrub the gourd with soapy water. I use dish soap and a heavy-duty scouring pad to clean the gourd. You can then paint it with acrylic paint, stains, or whatever. Make sure you use a good protective finish if you're using it outside.
Some people are sensitive to the dust from the gourd.
There will be dried pulp and seeds inside the gourd.