It is like comparing with apple and oranges but let me try to answer this -
GTP is GPRS Tunneling Protocol which provides tunneling over IP and used to carry signaling and data between GRPS/UMTS/LTE network nodes.Check the LTE architecture http://tech.queryhome.com/27610/protocol-architecture-of-lte-network
GRE is more generic i.e. the name itself is Generic Routing Encapsulation. It is also a tunneling protocol (encapsulate a wide variety of network layer protocols inside virtual point-to-point links over IP). .