Samsung has a "feature" called App Power Monitor. It aims to save battery by killing apps as it sees appropriate. Unfortunately, what it thinks should be killed isn't always what you think should be. Here's how you can prevent App Power Monitor from killing Navigation Gestures.

The first thing you'll want to do is open Settings. You can do that either by finding it on your launcher, or by opening the notification center and tapping the gear in the top right.

Once you've opened Settings, find the "Device maintenance" entry and tap it.

Let it all load and then tap on the "Battery" tile.

If you have App Power Monitor activated, you should see a short list of apps. Scroll through these until you see "Unmonitored apps" and tap it.

This section contains apps that App Power Monitor doesn't observe or kill. Scroll to the bottom of the list and tap "Add apps."

Find the "Navigation Gestures" entry and make sure it's selected.

Finally, tap "DONE" in the top right, and App Power Monitor will now ignore Navigation Gestures.