I've connected my GTOCP4 with wired ethernet (vs wireless). I determined the IP address that was assigned to it via DHCP by looking at the attached devices in my router's admin screens. You can also determine it using the technique described in the GTOCP4 Operating Instructions on page 56 in the "Determine Your IP Address" section.
Once I determined it's IP address, I entered it in a browser. That brought up the GTOCP4's Web Interface. I clicked on the "Advanced Ethernet Settings" button which brought up another web page that allows one to change the Wired Ethernet Properties. I set the DHCP Mode to "OFF" and then set the Default IP Address to the address I wanted the GTOCP4 to have which in my case is 192.168.1.110. I set the Default Subnet Mask to "255.255.255.0" and set the Default Gateway to that of my router, which is 192.168.1.1.
I almost messed up my GTOCP4 by actually mistakenly typing the wrong IP for the default address! After panicking for a few minutes, I was able to get back to the Web Interface to fix it by attaching a laptop ethernet port directly to that of the GTOCP4, which served as an Access Point. The laptop got an IP address from GTO and I was able to determine the IP of the GTO and was able to open the Web Interface to correct my mistake.
After rebooting the GTO, I was able to connect to it with APCC using the IP address.
My Comcast router has a feature where I can tell it I assigned a static IP to the GTO. I did that to ensure the router didn't assign the GTO's IP to any computer on my network via DHCP.
Hope that helps!