Does a iphone 8 gsm cdma unlocked need a sim card to activate?

Brand new out of the box?

Yes. As I told you in your other question, ALL iPhones need to have a SIM card in them for the activation process. It is only required one time. After the activation you can take out the SIM card. The phone will occasionally nag at you that there's no SIM installed, but it will still do all other non-phone related tasks with no issues.

It doesn't matter if the phone is unlocked or not; it will need a SIM at least once.

Yes. All iPhones do.