Both, the recipient and the giver are liable for the German gift tax (Schenkungssteuer).
The taxation applies if either the recipient or the giver is qualified as a German resident person. A resident person is"
I know here in the US one can gift up to a certain amount each year per individual without gift tax liability. Maybe this is what it is? Either way, this notary needs to tell you what they need and provide any proper forms or your signer needs to find out and get the proper forms. The apostille only confirms your commission and that you are a notary in good standing.
