Once we removed the old gable metal, it was obvious that the water had been filling up under the gable metal and soaking the fascia and back, flowing into the roof, causing the leak inside.

Here are a few before pictures and after pictures of the replaced wood.
Here are a few pictures after we replaced the wood.

In these pictures, you can see that the trim metal we used goes under the metal roofing, allowing water to go down the roof, out onto our drip edge, and off the roof, versus down the roof and under the other metal.