We've been doing some research for many hours already.
The first solution that appears everywhere (other StackExchange questions too) is using third-party service SendGrid.com and sometimes in conjunction with Zapier, but this it's not the approach we are looking to implement.
For sending without third-parties in the middle, the first solution that seemed viable was "Firebase Triggers", which was anounced on Google I/O 2014 but never came to life and seemingly was merged into Google Cloud Functions which is still in Alpha.
I'm pretty sure I had seen the solution almost a year ago in a link on the FireBase blog, but the only post that seems to have existed upon that matter is now empty.
We've never worked before with GCP, but our logic tells us this matter should be solved (inside Google) using with another GCP existing API, and the Mail API is apparently the correct one, but there seems not to be any way our Firebase Web App could make the request.
Could anyone (preferably with GCP experience) please explain what's the situation here, and how does Google expects it's FireBase developers to send emails to their clients?