What is "the gift of God" in Ephesians 2:8?

The following statements are excerpted from the classic book Principles of Interpretation, by Dr. Clinton Lockhart.

In Eph. 2:8, Paul says, “By grace have ye been saved through faith; and that not of yourselves: it is the gift of God.” We may ask, what is the gift of God? Many would answer, “grace;” many others, “faith;” some, “salvation.” But what does the grammar require? In the Greek, the words for “grace” and “faith” are both in the feminine gender. The pronoun “it” is not in the Greek, hence “it is” are in italics in the English Bible to show that these words are supplied by the English translators; but “it” is the same thing as “that” in the clause “and that not of yourselves;” and “that” in the Greek is neuter gender. Greek grammar requires that a pronoun should agree with its antecedent in gender; according to which the word for neither “grace” nor “faith” can be the antecedent of “that,” which shows that neither of these is the “gift of God.” The only other possible antecedent is the salvation expressed by the verb “saved.” Some have objected that the Greek noun for salvation is feminine; but we must notice that salvation is here expressed, not by the noun, but by the verb, and Greek grammar again requires that a pronoun which refers to the action of a verb for its antecedent must be neuter. This exactly suits the case; and the meaning is, Ye are saved by grace through faith; but the salvation is not of yourselves, it is the gift of God. Here the interpretation that accords with the grammar is reasonable and satisfactory.[1]

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[1] Clinton Lockhart, Principles of Interpretation, Revised Edition (Gospel Light Publishing Company, 1915), pp. 85-86, italics his. In regards to the Greek grammar of Ephesians 2:8, A. T. Robertson similarly writes: “And that (και τουτο). Neuter, not feminine ταυτη, and so refers not to πιστις (feminine) or to χαρις (feminine also), but to the act of being saved by grace conditioned on faith on our part. Paul shows that salvation does not have its source (εξ υμων, out of you) in men, but from God. Besides, it is God's gift (δωρον) and not the result of our work.” (See A. T. Robertson, Word Pictures in the New Testament, vol. 4, p. 525, comment on Ephesians 2:8.)

Editor's Note: The librarything.com website give this short biography of Dr. Lockhart, who lived from 1858-1951: “Graduated from Kentucky University 1885. PhD from Yale in 1894. Professor University of Michigan 1901. President of Texas Christian University from 1906-1910. Dean of TCU in the 1920s. Clinton Lockhart was president of the Christian College in Columbia, Kentucky and the Christian University in Canton, Missouri; he later taught Semitic and Biblical literature at Drake University.” (https://www.librarything.com/author/lockhartclinton)