My approach on the Gapped Kakuro was actually center before left, which meant I didn't have the benefit of knowing the 6 in the middle row.

What I noticed is that the columns of 40 and 43 mean that you need two 9s and two 7s in those columns. The 16 accounts for one of each, and the 15 accounts for at most one of the other. So where's the fourth? It turns out you can eliminate it from all the rows except for the 2-cell 9 clue -- and putting the 7 there leads to a contradiction.