The gauge will handle being on the tank's drain but flush the thing first, or you will probably clog the gauge with sediment when you open the drain valve. It's you that may have problems with the heat, so would need gloves or pliers to maybe install or remove it.
You can often pick up an adapter for a sink aerator, take the aerator off, then screw in the adapter then install the pressure gauge to it.
The cable's in the way, is that a T&P valve on the top of the WH? It should be. It will have a test, release lever on the top and probably a tag.
A pressure relief valve inline to a WH without an expansion tank is a problem waiting to happen.
I do not know how common closed supply systems are in Canada, but if you have one, and do not have an expansion tank, you should add one, otherwise, there is no way you'll ever stop that valve from releasing pressure while the WH is running...water expands when it is heated, and if there's no place for it to go (a closed system means it can't push back out into the supply, which is one reason they make them, to protect the water supply to others in case yours became polluted some way), the pressure WILL rise, and the pressure relief valve is just doing what it should be. An expansion tank, properly precharged, prevents that pressure rise. You'll need a pressure gauge to determine the proper precharge on an expansion tank. With one, you could probably get rid of the pressure relief valve, and install the expansion tank in its place.