The Collaborative International Dictionary of English v.0.48:

Gre \Gre\, n.
   See Gree, a step. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gre \Gre\, n.
   See Gree, good will. [Obs.]
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

GRE
       Generic Routing Encapsulation (RFC 1701, VPN)


The Free On-line Dictionary of Computing (30 December 2018):

Generic Routing Encapsulation
GRE

    (GRE) A protocol which allows an
   arbitrary network protocol A to be transmitted over any other
   arbitrary network protocol B, by encapsulating the packets
   of A within GRE packets, which in turn are contained within
   packets of B.

   Defined in RFC 1701 and RFC 1702 (GRE over IP).

   (1998-07-19)