The death estate includes all assets held at the date of death. What you refer to as death gifts are probably legacies left on death (these will be mentioned in a person’s will). Some are exempt (spouses, charities,political parties).
If the legacy is to a chargeable person, the IHT is borne not by the specific legatee who received the asset but by the residuary legatee (i.e. who receives the residue of the estate).