There are several possible reasons why CICS® Transaction Gateway is not able to connect to CICS over SNA; for example there might be a problem with an SNA server, or there might be a CICS Transaction Gateway configuration problem. Alternatively, there might be a problem with the connection definition in CICS.
CICS Transaction Gateway cannot connect to CICS over SNA.
For information on configuring the Client daemon to use SNA connections see Configuring an SNA CICS Server definition and Defining SNA connections on CICS Transaction Server for z/OS.