A gift tax can be simply defined as the tax imposed upon the individual giving any value to the other individual. It may be defined as a tax imposed on the value of a gift.
The person who gives the gift generally pays the gift tax in most cases. So it is a tax on the monetary gifts to another person.
Some times a gift tax can be defined as a transfer tax imposed on the gifts.
In the United States, the practice of Gift Tax is such that the tax is imposed on monetary and non-monetary properties. The income tax practice in the US is such that most taxes are excluded from the gross income.