There are a handful of threads on this already. Depends on each rig and ya never know exactly, but AFAIK everyone here chiming in found a HOT wire in there, no switch. The Gateway comes with a small in-line switch and someone, don't recall who, posted a nice few pics in that other thread of how to use a dremmel-like tool to cut out a corner of the Gateway and have the switch available from the outside.
I mounted mine inside for expedience-sake and simply have to pull the Gateway off the mount to flip the switch.
I'm using it only for 4G boost for my Verizon Jetpack, so only turn it on when I need it, and I haven't needed it yet, FWIW.
However, if you add a sim card and make the Gateway your actual wifi device, then you'd probably want a more accessible power switch.