Option to count unmeasurable gaps as sleep

I do not think  they will ever do so. For two reasons:

1. Marking unmeasurable periods as sleep would be more misleading than marking them as what they are, like it is now. If they marked a period that is unmeasurable as sleep, you might get false positive feedback about your health, which would be potentionally much more dangerous than telling you that there is an umeasurable gap in your sleep, and it is then up to you to decide how to interpret it.
   
   
2. Garmin, when developing their devices is bound by contracts with insurance agencies who increasingly use fitness devices for the calculation of health benefits to clients willing to use the fitness monitoring. In such case any assumption like the one you propose are not acceptable.

Better than marking the gaps as sleep, would be finding out why it happens in your case. There are several possible reasons - for example bad fit of the wrist band. A common reason might be also sleep apnoea, which induces strong vasoconstriction due to hypoxia. And with a strong vasoconstriction (limited bloodflow in the extremities), the OHRM of the watch cannot function properly.

You can try finding a better fit of the watch, or better position - another hand, inner side of the wrist, higher on the wrist, or using the ankle instead of the wrist, but if you suffer sleep apnoe or another heavy sleep disorder, using a chest HRM band is probably the only reliable solution.