Here "grace" refers to God's choosing Paul to be an apostle and leader of the church. You can make this explicit in your translation. You can also translate this in an active form. Alternate translation: "because God freely chose me to be an apostle, I can say" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-activepassive]])
Paul implies here that believers have different abilities that correspond to their faith in God. Alternate translation: "since God has given each of you different abilities because of your trust in him" (See: [[rc://en/ta/man/translate/figs-explicit]])