There should be no problem with this procedure on an Apple IIGS. You are
invoking the Terminal mode from the Super Serial Card firmware, which should
work.
The firmware may not be in modem mode. If this is the case, the method should
be Control-I T instead of Control-A T. (Control-I T is the command for the
Printer/Terminal mode of the firmware, Control-A T is for the Modem mode.)
Access II does work on the Apple IIGS if version 1.2 is used.
Another terminal package is "ASCII Express MouseTalk" from United Software
Industries.
Shareware terminal packages are available from various bulletin boards, but
without a terminal package, accessing a bulletin board would be difficult
at best.
For more information, search under: "United Software"