Unfortunately not. You can only connect FulGaz to either your smart trainer or your power meter, not both.
If you connect power meter you will be able to ride FulGaz, but only using the standard slope resistance from your trainer. Your on-screen speed will still be dictated by your power output, but you won't experience any automatic gradient / resistance changes. You will not be able to use ERG mode in workouts, as FulGaz will not be able to automatically "control" your trainer's resistance.
For the very best FulGaz experience, we recommend choosing your smart trainer as the Power & Trainer Control source.