Hi there,
Are you trying to send an SMS in a context of established SIP call (session)? Not sure why to do it this way; usually SIP MESSAGE is sent as unsubscribed out-of-dialog message. In HMP terms it means that the message is sent from board level device handle (obtained by gc_Open(:N_iptB1:P_P_IP)), which is denoted as GCTGT_GCLIB_CHAN. The message body should be placed in MIME part of this message as a "Content-Type: text/plain"
The bad news, however, is that even though this message (SIP MESSAGE) is defined in the header files, it is not yet described in the docs, neither supported by Global Call, as far as I can tell from my quick test. I'll see if I can get a latest status on it and will let you know.