How to use an unlocked gsm iPhone on a cdma network like virgin mobile?

You can't. GSM and CDMA aren't really compatible with each other.

"Every" wireless carrier supports LTE (4G) in some form, and so do most recently made phones, so some phones can be supported by ignoring the older wireless technology the phone was made for. Visit the carrier's "bring your own phone" page to see if your phone is compatible. If it is, it will work in areas where the network has built out their LTE-ready towers.

(Spoiler alert: for this, Virgin supports iPhones only and you should still check to be sure.)