The word “repentance” (”μετάνοια/metanoia” in the Greek Scriptures) simply means a change of one’s mind. Under the Gospel of the Circumcision it can technically include a change of actions as well, but under Paul’s Gospel it simply means that you’ve changed your mind about being able to do anything at all to contribute to your own salvation, and have realized that only what Christ accomplished can (and did) save you (and everyone, at least from an absolute perspective).
Basically, when you truly believe the Good News of the Uncircumcision, it means you’ve repented of your previous mindset that you could help save yourself through your own works, meaning through any action(s) that you might think you have to do in order to be saved — even the work/action of having to choose to believe something specific, or having to choose to have faith that the Gospel is true, which is probably the hardest work one could possibly do, if it were even possible at all — because God has now given you the faith to believe that Christ already took care of salvation for everyone through His death for our sins, His entombment, and His resurrection on the third day.
Basically, if God gives someone the faith to believe that everyone will eventually experience salvation because of what the truth of Paul’s Gospel means (because of what Christ accomplished because of His own faith, in other words), it means they’ve also repented, because their mind has now been changed about the Gospel (which, of course, also means they’ve been saved from a relative perspective and will get to enjoy the ultimate experience of that salvation before the rest of humanity does).
Meanwhile, those who haven’t come to believe the Good News of everyone’s salvation because of what Christ accomplished haven’t actually been given “saving faith” by God, which means they also haven’t repented and have not been saved (from a relative perspective, under Paul’s Gospel anyway) and will likely miss out on eonian life. Of course, in the end, they’ll still get to experience the salvation that Christ already won for them from an absolute perspective, but they’ll miss out on a lot in the meantime. (If you aren’t familiar with the different perspectives when it comes to salvation, please read this for more details.)