If I'm changing the input (and output) hoses in my gas water heater, do I need to drain the tank first? Or can I just shut off the water, shut off the gas to water heater and disconnect both hoses? I'm somewhat worried the system is under pressure and water will come shooting out uncontrollably...
All water on the planet is under pressure of its own weight by gravity.
If there are shut off handles for both the input and output on the house-side of the hoses, then shut off the input and the shutoff where water enters the house. Then turn on hot water in a sink. That will relieve any air pressure in the system.
If there is only a shutoff for the hose going TO the water heater, then you won't be able to stop all the water in the hot water pipes above you from running down and making a mess if you don't drain them, THROUGH the water heater.
If it were me, I'd shut off the gas and drain the water. It's not that hard. It gives you a chance to clean the tank out. If you end up having to move it it will be possible empty.