The Granville Sharp Rule in the New Testament

That rule says that when two nouns are joined by KAI, and the first is preceded by the article, they refer to the same person if:

1. both nouns are singular
2. both nouns are personal
3. both nouns are not proper names.

Since the nouns in Eph. 4:11 are not singular, pastors and teachers are not the same gift here.

See Dan Wallace - Intermediate Greek Grammar