Due to the different protocols that Apple use to transmit data, Apple Watch heart rate is not able to be picked up by the FulGaz app directly, there is a workaround for this though.
Using your Apple Watch as a Bluetooth Heart Rate monitor:
FulGaz can't communicate directly with your Apple watch, but you may be able to link it via a third-party app. You will need an iPhone to run the app and a separate device for FulGaz (iPad, Apple TV, laptop, etc).
Install HeartCast on your iPhone -> https://www.heartcast.app
Pair your Apple Watch with HeartCast.
This transmits heart rate from your Apple Watch via a Bluetooth channel from your iPhone.
Your Apple Watch HR should then appear as "HeartCast" under the Heart Rate option.