I work at a company that makes a group chat app. It includes games for group chats that celebrate the festivals of disbelievers. I doubted the permissibility of this job, so I resigned. I still have one more paycheck. What should I do with it? How do I repent for this job?

If it includes things like Christmas and other similar things, you could have worked on other things rather than these. But if you see that it is filled with that which is haram and you just left, alhamdulillah, you can take the money that you worked for and look for a new job.