And I saw another ANGEL flying in midheaven, having an ETERNAL GOSPEL
It is an ANGEL this time flying as only angels can do in midheaven preaching an ETERNAL GOSPEL. The ETERNAL GOSPEL means it has existed for all time, even before the fall of man. Perhaps it's this ETERNAL GOSPEL that God is explaining to Adam when He walked with him (Genesis 3:8-13). Now as then, the moral code passed in this ETERNAL GOSPEL is CHOICE. The CHOICE between 'knowledge of good and evil' (Genesis 3:1-7).
Recall that another angel, Satan, deceived man into sin by man's CHOICE. This 'another angel' says that judgment for those sins is near. Soon earthlings must 'FEAR GOD, GIVE HIM GLORY, AND WORSHIP HIM' (next verse) or ...... the CHOICE is in verse 9.
Now this ETERNAL GOSPEL does not set aside the 'gospel of the grace of God' (Acts 20:24) but rather embraces it. The CHOICE during this time of grace still is yours, 'Accept Christ (good) or Satan (evil)'. However, with the uptaking of the believers (1 Corinthians 15:51-58), the CHOICE comes down to taking the 'MARK of the BEAST' (evil - verse 9) or refusing it. This ANGEL is warning that the judgment time is very near for those who CHOOSE the MARK.
to PREACH
The command to PREACH was first given to us believers (Mark 16:14-18), but here in the final half of the tribulation, the message is so terrible (JUDGMENT/REAPING) that an ANGEL has to deliver it. The devil (AntiChrist) will be so oppressive that any persons who refuse to take his MARK will not dare expose themselves or talk about their CHOICE for fear of capture and certain death. However, the devil can't touch this angel since he is flying in midheaven. The devil has been tossed out of heaven (Revelation 12:9)
All who are alive will hear it as well as governments and leaders (see comment in Revelation 7:9).
Incidentally, this is the last time the word GOSPEL (good news) is used in the Bible. For earth people, particularly those who take the MARK, there is NO MORE GOOD NEWS!