FitBit does not allow SpO2 data, so this type of data must come from the Go2Sleep.
The external addon for this ring is designed to work only if selected as “Use wearable” (meaning no movement data).
But a few users were successful to pair the Go2Sleep as a BTLE device and read the data like it was an oximeter.
I cannot promise it will work, as the addon is not designed like that (and was updated almost two years ago, so I am not sure if it is not abandoned).
But the setup should be:
Settings > Sleep tracking > Wearables > Wearable > FitBit - for movement
Settings > Sleep tracking > Wearables > Smart BT - for Go2Sleep device HR
Settings > Sleep tracking > Wearables > Pulse Oximeter (Bluetooth) - for SpO2 data from G2S