Adam Clarke on Ephesians 6:19

And for me, that utterance may be given unto me - Ἱνα μοι δοθειη λογος. Kypke has proved by many examples that λογον διδοναι signifies permission and power to defend one's self in a court of justice; and this sense of the phrase is perfectly applicable to the case of St. Paul, who was an ambassador in bonds, (Eph 6:20), and expected to be called to a public hearing, in which he was not only to defend himself, but to prove the truth and excellency of the Christian religion. And we learn, from Phi 1:12-14, that he had his desire in this respect; for the things which happened to him fell out to the furtherance of the Gospel, so that his bonds in Christ were manifest in all the palace, and in all other places. Thus God had enabled him to make a most noble defense, by which the Gospel acquired great credit.

The mystery of the Gospel - The whole doctrine of Christ, not fully revealed previously to that time.