Writing this just post because searching the string "C++/CLI" const equivalent on a search engine did not find what I was looking for.

The answer is, by the way, "literal"

[Guid(MyClass::Guid)]
ref class MyClass
{
   public:
   literal String^ Guid = "6C46D25A-BB4C-4bf9-8B26-F78D98C4BC18";
}

(initonly which is the equivalent of readonly in C++/CLI will not work because it would not be usable as a parameter to an attribute).