The question is more opinion based. It all depends on your needs.

If your needs match one of below you should stay with MongoDB:

• Your app runs on-prem and contracts or privacy statements dont allow you to store data with a 3rd party.
• You need large storage but not much query power.
• There is other privacy/compliance issues.
• Your app does not have internet access (firewalls, isolated environments)
• You are running 3rd party applications that require a very old version of MongoDB

Here are some MongoDB Altas advantages:

• Easily deploy, modify, and elastically scale their database clusters with a few clicks or an API call
• Gain complete visibility into the performance the database and the underlying instances
• Focus more on development, with built-in operational and security best practices such as geographically distributed, auto-healing clusters, and always-on authentication and encryption.

The best way would be if you will check how work with MongoDB Atlas on GCP looks alike. You can check this tutorial.