It is permissible to trade gold-backed sukuk if there is a bilateral exchange at the time of the transaction.
The possession of both the sukuk certificate, and the currency, can be constructive rather than physical.
For the sukuk, possession can materialise by registering it under the name of the new sukuk holder.
For currency, this can be completed by a simple bank transfer at the time of the purchase.
And Allah knows best!
(Mufti) Billal Omarjee