GRE is a tunneling protocol designed to encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol internetwork. It's commonly used for creating VPNs, allowing compatibility with different networks, which makes it a suitable option for connecting the remote office with the main office securely. It is simple and creates fewer overheads as compared to more complex VPN protocols, hence the correct choice in this scenario. IPSec, on the other hand, provides robust encryption but is a more complex protocol that might not be necessary for all scenarios. SSL/TLS is primarily used for secure communication between applications, typically web browsers and servers, not for virtual point-to-point network tunneling. L2TP is used in conjunction with IPSec for VPNs and involves more complexity than GRE when setting up a tunneling VPN.