Please check this documents. It may be useful A beginner’s guide to skilling up for SAP Fiori and UI5
and SAP Fiori learning materials (2013 June edition)
Knowledge on ABAP knowledge, ERP project implementation experience ( HCM, SD, MM, Workflow, mobile, etc).
ABAP is prerequisite for a Gateway developer. If you dont know basic concepts of abap, you cant write odata services. Yes you can do to some extent without advance abap knowledge, but again how do you manage to write abap query. All this is required in order build an odata service.
If you are planning to be only a UI5 person, then you can stick to javascript and html5 only.
To develop a Fiori app : You should have knowledge on both Gateway and UI5.as a developer. SAP Basis and Security is required in order to understand the landscape and if you are a Technical architect.
Kindly let me know if you need any more information