Right now, gre(4) adds/removes encapsulating IP headers, it hooks
  into protocol switches in order to "catch" packets sent to its IP
  protocol numbers of interest, and it demultiplexes received packets.
  We can do all of that with a socket.  I will make gre use one.

I do not follow this.  Would one then have to have a user-space program
and a tun device?   Or would that then be possible, alongside the
current config?   Or did you mean something else?