Sounds like the washers/gaskets on the valve have gone bad, you'll have to either replace the washer or the entire valve (if you can find a replacement).

Here is what the valve may have looked like when it was new (just to give you an idea of what you're dealing with).

Removal

• Turn off the water to the shower faucet using either a local shutoff valve or the main water valve for the house.

• Turn the shower’s hot and cold water all the way on, to drain leftover water from the pipes.

• Pry the cap out of the center of the diverter’s handle.

• Remove the screw in the middle of the handle, and then pull the handle off the faucet.

• Unscrew the metal sleeve that sits over the valve stem.

• Insert a Shower Valve Socket Wrench over the valve stem and turn the dirervter anti-clockwise to remove it.

• Replace the washer, or the whole diverter if you can find a replacement.

• Apply some Silicone grease (plumbers grease) to the washer before replacing the diverter.

Installation

Installation is the reverse of removal.

• Insert a Shower Valve Socket Wrench over the valve stem and turn the dirervter clockwise to install it.
• Screw on the metal sleeve that sits over the valve stem.
• Push the handle on the faucet, then replace the screw in the middle of the handle.
• Push the cap into of the center of the diverter’s handle.
• Turn the shower’s hot and cold water all the way off.
• Turn on the water to the shower faucet using either a local shutoff valve or the main water valve for the house.

I don't think that you need to turn off the gas to the hot water heater. I believe that it is likely acceptable to leave the appliance on the 'pilot' setting.

I would obtain the manual for this device (try searching online, you will likely find it that way) or call the support number for the manufacturer.

However, what is acceptable practice is not necessarily best practice.

Personally, I would be hesitant to leave a gas appliance unattended for more than a day or two without physically disconnecting the gas (usually a 1/4 turn valve on the feed line). However, I feel comfortable lighting the pilot light myself.

Perhaps your should check with the local utility that provides your gas and see if they offer a service to visit your home and relight the pilot light upon your return? Mine does and it is free. My wife knows to contact them in the event I am unavailable because she is not comfortable at all in doing this herself.