According to TCB you can get cash back on items purchased with a gift card (I was sure I had done in the past.) They emailed this out today:
Payout with Zeek for a 20% bonus
We've teamed up with Zeek, the gift card marketplace, to offer you a 20% bonus when you turn your payable cashback into a Zeek eGift card. This means that if you have £50 payable cashback and opt to convert it all into Zeek credit, our 20% bonus will turn this into £60.
You can then use this Zeek credit to purchase gift cards from hundreds of leading retailers, such as Argos, NOW TV, Nike and more. The savings don't stop there though, the great thing about Zeek is that all the gift cards are discounted by up to 68%.
What's more, many of the brands you can buy gift cards for on Zeek are also on TopCashback, so you can earn cashback when you spend them too.