“Then I saw another angel flying directly overhead, with an eternal gospel to proclaim to those who dwell on earth, to every nation and tribe and language and people” (Revelation 14:6 ESV)

John saw an angel flying with the “eternal gospel” proclaiming it to everyone on earth during the time of tribulation. Even as the great majority of humanity had chosen to follow the antichrist, God still made sure the good news was heard, so that they were without excuse. This gospel is “eternal” because… (from Barnes Notes)
(1) “its great truths have always existed.”
(2) “it will forever remain unchanged.”
(3) “its effects will be everlasting” for those who believe.
It is this “eternal gospel” that God has given in order to rescue us from sin, separation and death.