You might have used HANA Cloud connector to access to OnPremise backend system (if any). Why dont you use same settings and create a new destination in HCP cockpit and access on-premise system in Web IDE.
Setting up SAP Web IDE with On-Premise System
How to configure an external GW system with SAP Web IDE
Regards,
JK
PS: I tried with connecting to gaas service after creating a new destination. it but didnt work.