Simple definition
A GRE tunnel encapsulates network packets inside IP to carry traffic between endpoints; GRE does not provide encryption on its own.
Purpose and use
GRE, Generic Routing Encapsulation, carries a packet inside another packet to connect networks through a tunnel.
How does it work?
GRE over IPv4 uses IP protocol number 47; this is not TCP or UDP port 47.
Example and verification
If the tunnel appears up but applications fail, check endpoint routing, protocol filtering and effective MTU.
Points to watch
Encapsulation adds overhead and provides no encryption by itself. Protection such as IPsec is required when the transport needs confidentiality.
In short
A GRE tunnel creates an encapsulated path; confidentiality, availability and loop prevention require separate consideration.