Does the GAP policy cover my road tax in the event of a total loss?

No; there is no need for it to do so because it would be refundable pro-rata by the DVLA if your vehicle is written off.

If your vehicle is declared a total loss by your insurer, you send section 9 of the vehicle logbook to the DVLA. You will then receive an automatic refund in the form of a cheque within 4 weeks.