I can't tell you not to use the immature gourds, but, you'll get much better seed from a mature gourd.
Choose a nice gourd of whatever varieties and save seed from them.
We call these gourds, "sacrificial gourds".

Jan

Sometimes a large immature gourd will have a few viable seeds if left on the vine until after the vine dies and is allowed to ripen (though it might rot instead of ripening) at 60-70F before drying.