You are looking for a quick solution to an issue without providing much in the way of feedback. You indicated that the phones are showing registered, but not where they show this. I assume at the PBX, but did you check to see what the phone itself shows? Did you try and get into the phones web interface and see what the phone shows? Did you check the firmware and upgrade if necessary?
Where did you buy the phones? Did you get them from an authorized GS reseller? If not, how do you know they are new? And again, what firmware version?
The situation appears to be that the phone is losing registration, but the PBX thinks it is still registered. When a registration occurs, the PBX will apply an expiration time to tell the phone when it needs to check back in and re-register. Once the registration is successful, neither the PBX or the phone will expect to hear from one another again regarding a registration until the expiry. So, at this point both devices are OK, however; if the phone goes off-line for some reason and can no longer communicate, the PBX will not aware of this and will continue to show the phone as being registered until such time as the registration expiry occurs and the phone fails to show up and re-register. This is why I wanted to see if you could get into the phone or ping it.
The keep-alives are designed to send a packet of data out to the SIP server at a given frequency to usually see if the SIP server is still alive and responding. In the process of doing this, it may also traverse a router to get to the server which in turn will cause the router to open a port to allow the traffic out and the router will keep the port open for a period in anticipation of a response. After a period with no traffic the port will close; hence why the keep-alive frequency setting can be sent such that the port remains open. On a LAN, there should be no reason to enable the option for any keep-alives as the switches should never block any traffic unless there is some issue with the switch or within the network, but ordinarily they should not block.