This is why all gutters need downspouts

As you can see, the gutter has no downspouts, so they left the ends wide open to allow water to escape. During heavy rains, water shoots out the back of the gutter, allowing water to go under the shingles, causing it to leak

the way to fix this is to cut the gutter to cut where I outlined it in red and put an end cap on the gutter to close it off. Then we would need to add a downspout going into the gutter bellow  

The red line in the photo below shows where the downspout will go