Unfortunately, this is likely to be hard. In C++, gRPC has its own event loop that it wants to use, and you generally can't run two event loops in the same thread. There are two options:

• Run gRPC in a separate thread, and use cross-thread communication (e.g. some sort of mutex-guarded data structure with kj::newPromiseAndCrossThreadFulfiller() for signaling) to communicate between them.
• Implement a custom kj::EventPort which wraps gRPC's underlying event loop, so that KJ code can operate inside gRPC's loop. This is non-trivial, but it has been done -- node-capnp, for example, implements an EventPort on top of libuv which successfully allowed Cap'n Proto to run inside Node. (Note: That code is quite old, and no longer works on current Node, but the capnp interfaces haven't changed, at least in 1.x.)