Is it possible to use genexus solely as backend? is that the case.. how it could be done?

Basically, you can develop data and service layers and use REST services to comunicate with UI. I've done sample applications against Android and web interface frameworks and it seems to do the job perfectly

Yes, you can use GeneXus for the Backend and create the Frontend in another technology. The simplest way is by creating services that work like an API. That in GX is pretty straightforward, exposing procs, or BC or Data Providers as webservices, with the technology you need. The API object was created precisely to solve these types of scenarios: https://wiki.genexus.com/commwiki/servlet/wiki?46151,Category%3AAPI+object With this object you can create a "layer" that is generated with the API, which would be the backend that the frontend that you use (React, Angular or whatever) is going to consume.