Hi Dhruv,
I guess you are talking about service maintenance in Service Builder(SEGW). If yes, then you are not able to view any systems under that where you can register and maintain your service. You are missing one configuration. Please follow steps below:
1. Go to SPRO transaction -> SAP Reference IMG
2. Navigate to node : SAP NetWeaver -> Gateway Service Enablement -> Backend OData Channel -> Connection Settings to SAP NetWeaver Gateway -> SAP NetWeaver Gateway Settings.
3. Maintain the corresponding Gateway hub system entry where you would like to register your service. Please note that while doing so you also need to create the corresponding RFC destination to that system.
Best regards,
Aakash