26 C.F.R. § 25.2502-2
Section 2502(d) provides that the donor shall pay the tax. If the donor dies before the tax is paid the amount of the tax is a debt due the United States from the decedent's estate and his executor or administrator is responsible for its payment out of the estate. (See § 25.6151-1 for the time and place for paying the tax.) If there is no duly qualified executor or administrator, the heirs, legatees, devisees, and distributees are liable for and required to pay the tax to the extent of the value of their inheritances, bequests, devises, or distributive shares of the donor's estate. If a husband and wife effectively signify consent, under section 2513, to have gifts made to a third party during any "calendar period" (as defined in § 25.2502-1(c)(1) ) considered as made one-half by each, the liability with respect to the gift tax of each spouse for that calendar period is joint and several (see § 25.2513-4 ). As to the personal liability of the donee, see paragraph (b) of § 301.6324-1 of this chapter (Regulations on Procedure and Administration). As to the personal liability of the executor or administrator, see section 3467 of the Revised Statutes (31 U.S.C. 192 ), which reads as follows:
Every executor, administrator, or assignee, or other person, who pays, in whole or in part, any debt due by the person or estate for whom or for which he acts before he satisfies and pays the debts due to the United States from such person or estate, shall become answerable in his own person and estate to the extent of such payments for the debts so due to the United States, or for so much thereof as may remain due and unpaid.
As used in such section 3467, the word "debt" includes a beneficiary's distributive share of an estate. Thus if an executor pays a debt due by the estate which is being administered by him or distributes any portion of the estate before there is paid all of the gift tax which he has a duty to pay, the executor is personally liable, to the extent of the payment or distribution, for so much of the gift tax as remains due and unpaid.
26 C.F.R. §25.2502-2