Where-is-the-battery-in-a-gmc-terrain Problem

It is behind the left headlamp under the Powertrain Control Module (PCM). You have to partially remove the front bumper fascia and the PCM to get at it. (see diagram)

If you are trying to jump another vehicle there are remote terminals so you dont have to remove the PCM but otherwise the diagram above is accurate however annoying that may be

BATTERY UNDER FLOOR PANEL UNDER FLOOR MAT BEHIND DRIVER SEAT.