yah, your approach is the approach i tried as well, and i'm seeing the same "not a Math::BigInt::GMP object" business. Devel::Peek shows some interesting details, but i don't know this stuff deeply enough to interpret it: 
0 dkg@pip:~$ perl -MDevel::Peek  -e 'use Math::BigInt only => "GMP"; m
+y $x = Math::BigInt->new(34); printf("%s\n", Dump($x));'
SV = RV(0x907fe54) at 0x907fe48
  REFCNT = 1
  FLAGS = (PADMY,ROK)
  RV = 0x9065818
  SV = PVHV(0x906ad24) at 0x9065818
    REFCNT = 1
    FLAGS = (OBJECT,OVERLOAD,SHAREKEYS)
    STASH = 0x90a0f30    "Math::BigInt"
    ARRAY = 0x90872d8  (0:6, 1:2)
    hash quality = 125.0%
    KEYS = 2
    FILL = 2
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "value" HASH = 0x1e720953
    SV = RV(0x907fe24) at 0x907fe18
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x90656f8
      SV = PVMG(0x90e21a4) at 0x90656f8
        REFCNT = 1
        FLAGS = (OBJECT,RMG)
        IV = 0
        NV = 0
        PV = 0
        MAGIC = 0x9097ba0
          MG_VIRTUAL = 0xb781aea0
          MG_TYPE = PERL_MAGIC_ext(~)
          MG_FLAGS = 0x10
          MG_PTR = 0x9083500 ""
        STASH = 0x91ab538    "Math::BigInt::GMP"
    Elt "sign" HASH = 0xe7455c8f
    SV = PV(0x9063748) at 0x9065998
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x90833b0 "+"\0
      CUR = 1
      LEN = 4

0 dkg@pip:~$

[download]
And here's the same thing with Math::GMP: 
0 dkg@pip:~$ perl -MDevel::Peek  -e 'use Math::GMP; my $x = Math::GMP-
+>new(34); printf("%s\n", Dump($x));'
SV = RV(0x868ce44) at 0x868ce38
  REFCNT = 1
  FLAGS = (PADMY,ROK)
  RV = 0x8672998
  SV = PVMG(0x86cc918) at 0x8672998
    REFCNT = 1
    FLAGS = (OBJECT,IOK,OVERLOAD,pIOK)
    IV = 141099448
    NV = 0
    PV = 0
    STASH = 0x86adf00    "Math::GMP"

0 dkg@pip:~$

[download]

In reply to Re^2: accessing mpz_t * of Math::BigInt::GMP from XS by dkg
in thread accessing mpz_t * of Math::BigInt::GMP from XS by dkg

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