binding opaque C objects with luajit ffi metatype

Question

In the ffi documentation, it mentions that " the C type metamethod mechanism is most useful when used in conjunction with C libraries that are written in an object-oriented style. Creators return a pointer to a new instance, and methods take an instance pointer as the first argument. Sometimes you can just point __index to the library namespace and __gc to the destructor and you're done. But often enough you'll want to add convenience wrappers, e.g. to return actual Lua strings or when returning multiple values."

I have a C library that looks like this:

    local ffi = require("ffi")
    ffi.cdef [[
        typedef struct T *T;

        T new_t();
        void t_set(T t, int value);
        int t_get(T t);
        void t_free(T t);
    ]]

I'm binding it in the following way:

    local lib = ffi.load("./lib/libtest.so")
    local tLib = ffi.metatype("T", {
        __index = {
            new = lib.new_t,
            set = lib.t_set,
            get = lib.t_get,
        },
        __gc = lib.t_free
    })
    local t1 = tLib.new()
    t1:set(10)
    t1:get()
    t1 = nil

Why doesn't setting t1 to nil invoke the __gc metamethod? Also, is my approach correct for binding opaque C objects?