OK, found it on Westermo Web.
It looks like the GS-01 is now a discontinued item (according to Westermo web site). However, setting this up shouldn't be a problem.
I would suggest using the PLC Settings to set the PLCs Host Link port to default settings - "Host Link", 9600, 7 E 2.
Then, if the modem(s) at each end are configured likewise, there shouldn't be a problem.
Remember in the project setup, to select SYSMAC WAY as the Network type, (with baud and data format set as above), and to ensure that the modem details are correct (including area codes etc).
Here is a config string that can be downloaded to a remote GD-01, to enable comms with the PLC -
AT&F+IPR=9600;+IFC=0,0;+WRST=1,"035:35";S0=1E0&D0Q1&W
(NOTE - I don't know if this is the same for the GS-01. Best talk to Westermo. They are pretty knowledgeable about communicating with Omron kit, and I'm sure that someone there can help you.)
Hope this helps a bit