Gentile Christians. Trusted, which is not in the Greek, is unnecessary. The pronoun ye is nominative to were sealed.
In whom
Resuming the in whom at the beginning of the verse, and repeated on account of the length of the clause.
Ye were sealed (ἐσφραγίσθητε)
See on Joh 3:33; see on Rev 22:10. Sealed with the assurance of the Holy Spirit. Rom 8:16; 2Co 1:22; 2Ti 2:19.
Spirit of promise
Strictly, the promise. Denoting the promise as characteristic of the Holy Spirit: the Spirit which was announced by promise. See Act 2:16 sqq.; Joe 2:28; Zec 12:10; Isa 32:15; Isa 44:3; Joh 7:39; Act 1:4-8; Gal 3:14.