I have followed the instructions here: https://github.com/grpc/grpc-kotlin/tree/master/compiler for Gradle, and everything is working fine, but IntelliJ is seeing errors in the generated code, even though when I run the application and manually write code I know would work it runs just fine.
Intellisense was disabled in one of the generated code files because of file size, causing all the others to have errors. I was able to increase the intellisense max file size and now everything works!