There are probably other ways to save seed, but our method has worked for us......

choose any gourd you want while it is still green and on the vine, but near the end of its growing season. (we call this the sacrificial gourd)

cut it open and separate the seeds from the gourd guts.

spread the seed on newspaper in a cool dry place.

check it once a day and stir it to separate the seeds and detach them from the paper.
when they are very dry (about a week) put them in labelled freezer bags and freeze them.

Of course, you are going to have to label them Mystery Gourds, as you said, your vines are ALL MIXED UP!
Jan