I couldnt find anything about this: Is Secure Private Access (on-prem) wroking/supported with Unified Gateway on the NetSclaer?

You can do on-prem SPA and that works with NetScaler. SPA service only leverages gateway service

I mean that "Unified Gateway" vs "NetScaler Gateway"

Yes, as long as you are using advanced policies, which are a prereq for ACVPN, which is a prereq for SPA On-Prem