I don't know what XMPP priority the Simonics GVGW is using; again, the reason your devices aren't ringing is because the GVGW is grabbing the priority. Rebooting the device(s) is simply and temporarily giving the last device to connect to the Google XMPP server the priority, but it will be superseded as soon as GVGW re-connects.
You would have to a) find out from Bill Simon what priority GVGW is using, and b) if it the highest, then lower it to some level, and then c) enter a higher level on the OBiTALK portal for your OBi, in "expert configuration" mode, and then d) write this down, so you don't come back here in a year and ask how to do it again. I have not tried this myself, so, while it may work in theory, the only way to prove it works is to try it.
From the OBiTALK dashboard, click on your OBi 20x device(s). Get past the various links and buttons and warnings, to get into expert configuration mode, then go to "Voice Services", and click on the SPx that you are using for Google Voice (SP1--SP4), then scroll down to the calling features section, then remove BOTH check marks to the right of the following value, and then enter the desired priority, and then click "Submit" at the bottom of the page.
See the screenshot below.