Does Verizon iPhone 6S Plus support both CDMA and GSM network?

Yes. It supports both CDMA and GSM, however, many non US CDMA carriers use what they refer to as a "CDMA SIM". It will not work on those networks.

It supports both GSM and CDMA networks, however unlocking really only applies to GSM.

Once unlocked it will work with any GSM network's sim card.

However, CDMA carriers would need to activate it on their CDMA networks and many do not do this.

It technically is capable of connecting to CDMA networks, but it will be up to the receiving carrier to actually activate it on their CDMA network.

As Phil explained, it is theoretically capable of supporting any CDMA network that doesn't use a "CDMA SIM". Whether any given CDMA carrier will allow the phone on their network is something you'll need to ask them about. Many CDMA carriers will only allow on their network phones that they sold.

Verizon phones are sold as unlocked. There is no need to get them unlocked. Note that "unlocking" only applies to GSM/LTE. It has no meaning for CDMA.

You're most welcome.