You can run MemSQL (SingleStore DB) on GCP. SingleStore Managed Service runs on there, or you can self-host clusters on there.
There is no open source version of SingleStore DB. The free version has all the features of the enterprise version. There’s no size limitations on the enterprise version of any kind. Check our website for limitations on the free version.
Your question about read thru is not clear to me. SingleStore DB is a database system with full persistence. If you commit an update, it’s permanent.
Yes, you can create client applications in C++.
We don’t have any sort of built-in synchronization with Postgres or Mongo. You’d have to use a data integration tool or app code to do that.