The stty or ditty.dgap settings do not stay configured on the ports and revert to 9600/8/1/None whenever the ports are opened/closed or after the system is rebooted.

It is typical behavior in UNIX to revert port settings to default values when a port is opened/closed.

Create a startup file that is executed after the dgap driver module is loaded.

Here is an example, create and edit a file called:
/etc/rc5.d/S95dgap_ditty

The following entries, will configure the port for 4800 baud and software flow control for port 1:

cat < /dev/ttya01 > /dev/null &
ditty 4800 ixon ixoff -ixany ttya01

Link it to the /etc/init.d directory:

ln -s /etc/rc5.d/S95dgap_ditty /etc/init.d/dgap_ditty