if i remember right IntelliJ didnt see the generated sources until I explicitly configured them,

sourceSets {
    main {
        java.srcDir("${project.buildDir}/generated/source/proto/main/java")
        java.srcDir("${project.buildDir}/generated/source/proto/main/grpc-java")
        java.srcDir("${project.buildDir}/generated/source/proto/main/grpc-kotlin")
    }
}