I see, but I'm not sure I follow that understanding exactly.
I read "and as many as had been appointed to aeonous life, believed the gospel." I'll reword the declaration without changing the meaning...
"as many as had been appointed" and "believed the gospel" is an identity claim here, that is, a=a, they are one and the same subject. Paul is telling us that, "as many as had believed the gospel had been appointed to aeonous life."
Keep in mind that none of us will wholy realize aeonous life until the last day, when we all stand together in the resurrection. So we are, too, have been "appointed to aeonous life."
You did change the meaning. You moved the object of belief from what Paul had told them from God to something the text did not say Paul said.
For example, in John's gospel, when peter and John came to investigate Mary's theory that the soldiers or the Sanhedrin had taken the body of Jesus, Peter and John and found the tomb empty, It says John believed, but it does not say "John believed the gospel". In fact, the text makes plain that they did not yet understand that Jesus must rise from the dead, so John could not possibly have believe the gospel. The context infers that John believed what Mary had said, that the soldiers or the Sanhedrin had taken the body away.
20 The first day of the week cometh Mary Magdalene early, when it was yet dark, unto the sepulchre, and seeth the stone taken away from the sepulchre.
2 Then she runneth, and cometh to Simon Peter, and to the other disciple, whom Jesus loved,
and saith unto them, They have taken away the Lord out of the sepulchre, and we know not where they have laid him.
3 Peter therefore went forth, and that other disciple, and came to the sepulchre.
4 So they ran both together: and the other disciple did outrun Peter, and came first to the sepulchre.
5 And he stooping down, and looking in, saw the linen clothes lying; yet went he not in.
6 Then cometh Simon Peter following him, and went into the sepulchre, and seeth the linen clothes lie,
7 And the napkin, that was about his head, not lying with the linen clothes, but wrapped together in a place by itself.
8 Then went in also that other disciple, which came first to the sepulchre,
and he saw, and believed.
9 For as yet they knew not the scripture, that he must rise again from the dead.
In this story, we have the same kind of issue. The text says those who had been appointed to aeonous life believed. It does not say they believed the gospel. The immediate context tells us what they believed and were so elated about: it was what Paul had just told them. that He had been commanded to take the gospel to the Gentiles because the Jews had shown themselves unworthy of it.