This is not a problem!
If the total contribution amount falls short of the gift goal, the registry owner will receive a Target eGiftCard for the total amount collected. These funds can be applied toward an item in two ways:
1. The value of the gift card can be applied to this particular gift -- and the registry owner can subsidize the difference
2. Or, the value of the gift card can be applied to any item sold at Target or target.com
TIP: This approach holds true regardless of how many (or how few!) people contributed