Wanted to share an app that I am testing this week that allows you to ride Zwift on a gym / spin bike. The app is called HR2VP. It uses your HR and it estimates/converts your effort into watts and it transmits the estimated power via Bluetooth to Zwift.
It’s very easy to use. All you need is a HR monitor, the app on your phone, and Zwift either on your phone or tablet. It’s a handy way to ride Zwift while traveling when all you have is a gym.
I tested on a spin bike in the AirBnB I am staying, but the bike is way too old. I will try to go to a gym this week and do some more testing, but my first test worked out pretty well. I just wasn’t able to go hard just yet - to give you an idea, I was afraid of getting tetanus from the bike 
I’ll keep you all posted.