Below is from Sharetechnote
When I first saw the frequency specification called GSCN(Global Synchronization Raster Channel). I asked myself ‘why we need this kind of additional frequency specification like this when we already have the existing frequency spec(raster) called ARFCN ?’.
I still don’t know of the offical answers in 3GPP TS or TDocs (I failed to find the answers in the specification). So I just came up with my personal guess as follows :
When NR is activated in NSA, UE does not need to do blind search for SSB since the frequency, subcarrier spacing etc are all configured by LTE RRC Connection Reconfiguration message.
But in SA, UE need to blindly detect SSB since UE need to detect this before it gets any RRC (MIB, SIB). If UE needs to search SSB based on ARFCN raster, it would take too long time since ARFCN raster is very narrow. So it would be good to define a SSB searching frequency in wider steps. This is the usage/purpose of GSCN.
And below is my note from a training (more than a year ago)
If UE needs to search SSB based on ARFCN raster, it takes king time since ARFCN is very narrow. So, it would be better to define a SSB searching frequencies in wider steps. This is called GSCN (Global Synchronization Raster Channel)
Hope helps,