Hi Carlos,
There is more to HCP than just using it to build Fiori Apps. You can view some of the HCP capabilities here. From an application development perspective, you can also use Java/HANA XS to build applications and consume data from SAP and non-SAP backend systems too.
Yes, Gateway comes inbuilt with the recent SAP Business Suite systems and you could use it to build your Fiori Apps. One of the main advantages of using HCP in this case would be to decouple application lifecycle from the stable core SAP system. HCP provides Gateway as a service and Fiori Launchpad. SAPUI5 libraries get updated on HCP and there is no maintenance efforts required to patch the system. WebIDE is the development tool in HCP to build and extend Fiori Apps and it has its own benefits. As part of Fiori Cloud Edition, Fiori Apps are also being made available on HCP. This makes the implementation of Fiori Applications much more simpler.
I hope I answered your question.
Cheers,
Murali.