I'm not a lawyer and I can't answer any further. I know you are talking about "US situs" property, I've seen other web sites that say the "situs" argument doesn't exist. At worst, even if the gift giver has to file a gift tax return, there is a lifetime exclusion of at least $11.4 million, so even if a gift tax return is required due to the "situs" rule, no gift tax could possibly be owed unless the amount was more than $11.4 million. And if your family are gifting that much, you should be getting paid professional tax advice. Presumably, if they needed to file a gift tax return, they would also need to file a form W-7 application for an ITIN.
