Peter, too, talks about the gospel in his first letter. We begin in chapter one:

10 Concerning
this salvation, the prophets, who spoke of the grace that was to come
to you, searched intently and with the greatest care, 11 trying
to find out the time and circumstances to which the Spirit of Christ in
them was pointing when he predicted the sufferings of Christ and the
glories that would follow. 12 It
was revealed to them that they were not serving themselves but you,
when they spoke of the things that have now been told you by those who
have preached the gospel to you by the Holy Spirit sent from heaven.
Even angels long to look into these things.

The Old Testament prophets preached a glimpse of the grace of God in Christ.

They
evidently sensed it was only a glimpse because they searched and
yearned to discern what their grace message was ultimately pointing
toward.

That search, prompted as it was by the Spirit of God, was pointing both the death and resurrection of Jesus Christ.
That search is now over: Jesus Christ has arrived.
The readers of Peter’s letter had heard this gospel, that is, they had been evangelized. The Spirit was on those preachers.

This gospel is called “salvation” to open up this paragraph: the gospel, then, is salvation through the death and resurrection of Jesus Christ.