You have the GIA certification that they are natural pearls-- so yes, they are valuable!
You inherited it with a family member...I recommend you do not split up the pearls, or else you will need to recertify them. The certification is for the necklace as a whole.
If you plan to sell it, find out what the metal and stones are, as that can significantly affect their value. It's best not to assume; any pearls may be strung with any clasp.