To reproduce:
1. Open attached project
2. Build and run to any platform

Actual result: the Fatal error in Unity CIL Linker appear
Expected result: it should build without errors

Reproduced with: 2018.4.8f1; 2019.2.0f1; 2019.2.3f1; 2020.1.0a2
Can not check on older versions due to errors of missing packages

Note: User says that the issue does not happen on 2018.3.8 but can not be tested.

Error:
Fatal error in Unity CIL Linker
Mono.Linker.MarkException: Error processing method: 'System.Void Gryphon.Protocol.CustomMetadata/CustomMetadataDeserializer::Deserialize(System.ArraySegment`1<System.Byte>,Gryphon.Proto.IProtoBuf&)' in assembly: 'Assembly-CSharp.dll' ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Mono.Linker.MethodBodyScanner.GetReferencedInterfaces(AnnotationStore annotations, MethodBody body)
at Mono.Linker.Steps.MarkStep.MarkInterfacesNeededByBodyStack(MethodBody body)
at Mono.Linker.Steps.MarkStep.MarkMethodBody(MethodBody body)
at Unity.Linker.Steps.UnityMarkStep.MarkMethodBody(MethodBody body)
at Mono.Linker.Steps.MarkStep.ProcessMethod(MethodDefinition method)
at Unity.Linker.Steps.UnityMarkStep.ProcessMethod(MethodDefinition method)
at Mono.Linker.Steps.MarkStep.ProcessQueue()

Note:
"The assembly 'Gryphon.Proto' has a mismatched file name: 'Gryphon.Proto.1.0.0.29.dll'. This can cause compilation problems on some platforms."
As a workaround you can change the file name of Assets/Gryphon.Proto/Gryphon.Proto.1.0.0.29.dll by Assets/Gryphon.Proto/Gryphon.Proto.dll
Note: works fine on standalone platform