Not as such, no. Your server can (and probably does) run the OpenSSH daemon, which will listen on TCP port 22 for incoming connections. If the server is on a LAN, only hosts on that LAN can send packets to its IP. Therefore, only hosts on its LAN can initiate communicate with it.
TCP has two kinds of sockets: listeners an initiators. A listening socket sits around and waits for someone to send it a packet. An initiator opens a connection by sending a SYN packet to a listening socket. In SSH, the server is a listener and the client is an initiator. If the initiator can't get a packet to the listener, it can't open a connection. Thus you need to forward a port on the gateway (which the client can talk to) to the server (which the client otherwise can't see).