I'm not an expert on gRPC so take what I say with a grain of salt, but it does not appear to be a standard REST API and requires client libraries for whatever programming language you will use. 

I didn't see PowerScript but they do offer C# examples, which PowerBuilder supports.  So theoretically, you could call the C# implementation from PowerScript using the new .NET DLL importer feature of PowerBuilder 2019 R2.

But I would recommend you go with standard REST APIs simply for the fact that it would ensure that virtually any system developed with any programming language could readily interop with your business logic.  I think there is no other Web API standard as widely used as REST.

It seems the key scenario to use gRPC is if network bandwidth is a serious issue or you need to do bi-directional streaming.  You might find this tech article from Microsoft useful in comparing the differences and Microsoft's recommendation when to use gRPC.

In case you do decide to go with REST APIs, you may want to consider using the .NET DataStore to accelerate development.  There is little learning curve for most PowerBuilder developers, requires minimal coding with POCO models automatically generated for you, and it performs faster than popular frameworks like Entity Framework.