I'd call the plumber. Usually, the gas man is only concerned with whether or not the gas is getting to the heater - you can check that yourself. How old is your heater? and do you drain it all the way periodically? If not, sludge can build up in the bottom which can drastically reduce heating efficiency.
Try running your hot water in a sink until you think it's at its hottest; then take its temp. A meat thermometer works great. Around here plumbing codes recommend 120 Deg for the highest safe temp. If your thermostat is turned up max and you're getting less than that, you do have a problem. I'd still call the plumber first, especially if it's the one who installed the heater.
I'm assuming this problem has already been solved, but the answer depends on your local utility. Call them first. Some have their own maintenance people. Also, if your water heater is closing in on ten years old, it may be time for a new one. Sometimes the utilities can offer you a rebate on a new one. Also, some jurisdictions require a permit for installation, even replacements. Guess it's meant to keep us from blowing ourselves up.