Very good question & one that has seen much debate over the centuries. But in the Greek both faith & grace are feminine, whereas the “that” (touto, a demonstrative pronoun) is neuter, which means it is difficult to narrow down the gift of Eph 2:8 to either faith or grace. B/c of this many scholars believe the “gift” refers back to the previous statement about salvation described in Eph 2:4-8. Meaning that salvation as a whole is a gift. As Harold W. Hoehner explains, “[touto] refers back to the concept of salvation (Eph 2:4-8), whose basis is grace and means is faith. This salvation does not have its source in man (it is “not from yourselves”), but rather, its source is God’s grace for “it is the gift of God.” While John MacArthur says something very similar he adds, ““That” refers to the entire previous statement of salvation, not only the grace but the faith. Although men are required to believe for salvation, even that faith is part of the gift of God which saves and cannot be exercised by one’s own power. God’s grace is preeminent in every aspect of salvation.” While I personally agree with MacAthur, I admit that the Greek by itself does not demonstrate that faith is the gift Paul is referring to, but when considering that faith is certainly apart of the salvation process that Paul describes, it too can be considered a gift. But like I said—this certainly could be up for debate.