You’re on the right track! Your 2011 GMC Terrain’s battery location IS underneath the hood on the driver’s side, but it is also underneath your vehicle’s engine control module (ECM), which will need to be removed before you can access the battery itself.
You’ll first need to remove the EMC’s protective cover (which just snaps into place) and then use a 10 mm wrench to loosen the fastener holding the ECM in place. Once you remove the fastener, slide the ECM back, lift it up, and then move it out of the way—you should now be able to see the battery.