All unsold Gem listings expire after 36 hours, when your Gems will be automatically returned to your inventory.
If your Gems haven't been returned and you haven't received your Coins, your Gems are still listed. If you can not see your Gem listing, it's probably because of the 2.5% Marketplace selling fee.
We are working on improved Marketplace management features that will make this less confusing. Until then, trust that your Gems are still listed.