The Duke is no longer a member of the House of Lords (since the 1997 reforms), and no longer attends the state opening. Thus his wife cannot attend as a peeress. Before 1997 royal Dukes could meet in the House of Lords (even if they did not do so for political meetings), and their wives could attend the opening as peeresses, which is why you can find a lot of pictures of the Kent and Gloucester Duchesses from previous years.

The Duke and Duchess were both present (with the Earl and Countess of Ulster) on the gallery as guests because their grandson Lord Culloden was a page of honour to the Queen. The dress-code for guests on the gallery is not the same as for peers/peeresses.