Hello iamsoandso!
There is only one true gospel. The angel says:
"Fear God and give Him glory, because the hour of His judgment has come. Worship the One who made the heavens and the earth and the sea and the springs of waters.”
Who made the heavens and the earth and the sea and all things?
"For by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities-- all things have been created through Him and for Him."
The Father speaking about the Son:
"“In the beginning, O Lord, You laid the foundations of the earth, and the heavens are the work of Your hands. They will perish, but You remain; they will all wear out like a garment. You will roll them up like a robe; like a garment
h they will be changed; but You remain the same, and Your years will never end.”
And
"Therefore God exalted Him to the highest place and gave Him the name above all names, that at the name of Jesus every knee should bow,
in heaven and on earth and under the earth and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
Those great tribulation saints will be one of the groups on the earth during that time of which scripture states that they will have kept their testimony of Jesus and the word of God. And will not have worshiped the beast, his image, nor have received his mark.
The angel is preaching the same gospel (good news) message, which is to trust in Jesus for the forgiveness of sins, who created the heaven and the earth and the sea and all that is in them.