Normally you need blaze tier for outbound requests. However, if I use atlas on this: https://cloud.google.com/mongodb
can I still use spark (free) tier?
The URL doesn't look like a GCP URL (look like mongodb url) so I don't think it is whitelisted?
If it runs on Google Cloud Platform it should typically be whitelisted. But that's only on Node.js 8 and even then only until that is no longer supported (early 2021), as from Node.js 10 and up Cloud Functions will no longer be available on Firebase's free tier. See https://firebase.google.com/support/faq#functions-pricing for more on this.