does gif interface blocks/doesn't support packet fragmentation.

Do you mean fragmentation of the inner-AF payload within the gif into
multiple outer-AF packets, or creation of a larger outer-AF packet
which then gets fragmented.

IIRC, PMTU-D is mandatory in v6, and small links have to
fragment/reassemble at link layer (but I think this last point is
irrelevant here).

Not what you asked, but:
The 1500 byte packets arriving should have DF set.  gif(4) has no
support (that I saw when reading code or manpage) for copying DF to
the outer (such as IPsec has with the net.inet.ipsec.dfbit sysctl), so
I'd expect the creation of slightly larger than 1280 byte packets.
It would be interesting to consider propagating the effects of a v4
MTU back to v6 via setting outer DF.

  works until i try to send packet >1280, no fragmentation is performed i
  think - no reply.

I think the bug is not getting a icmp6 too big message, and this
should happen in ip6_forward.

-- 
        Greg Troxel <gdt@ir.bbn.com>