The PSU has nothing to do with what voltage is on the sync line.
If you got the specific one from RGC then it should be to spec and just plug-and-play
You can try checking voltage with multimeter (gives average so not perfect but gets you an idea of the level) or even better if you have access to oscilloscope.
Easiest is probably to contact Robert at RGC, he can help you.
If you get ”no sync”, and it’s caused by too low sync level for some reason, you can also try adjusting sync threshold on OSSC.