We have a gas water heater that is 15 years old. Is it normal for there to be a hissing noise coming from the relief valve? Also, the pipe that is hooked to the relief valve is hot. I don't remember it being this way before. Do I need to call a plumber?

this is an indication of abnormally high temperature,pressure,or the valve has failed.
call somebody.

Where do you live that they allowed that kind of piping for a T&P discharge?

We live in California. The pipe through the wall leads outside. And yes, I recently discovered that there's hot water draining out in our flower bed. A plumber is coming in the morning. Thanks.

Tell your plumber to re pitch the discharge piping so it drains properly as the picture shows it is slightly back pitched