The grpc repo doesn't provide a C language code generator or API. The core exposes a C89 semantically-versioned surface API but is implemented in C++11 (though much of it is in C style since the implementation was also historically C89 as well). Additionally, even the core surface is not considered stable; it is currently on the 7th major version while our actual APIs (C++, C#, Obj-C, Ruby, etc.) have remained stable since Beta or at least 1.0 in all cases.
That said, Juniper has offered a pure-C implementation and API at
https://github.com/Juniper/grpc-c . AFAIK, that repo is in a pre-alpha state but has gotten some use, and its maintainers are certainly part of the community (attended and presented at meetups, etc).