How can I go back to a gas water heater?
Only "power-vent" type gas water heaters can be vented with plastic pipe. These are gas water heaters with a fan on top which blows the flue gas (and cools it by mixing surrounding cool air with it, hence plastic vent is OK) out of the house. Most can be vented with 3" PVC (white), ABS (black), or CPVC (kinda tan colored). These vents can be run horizontally and can have 90 degree bends (limits on that though, see instructions/local code). So if the black pipe in question is plastic, it is probably ABS, and the vendor might be wrong.

Not sure if you know what you are getting into, direct-vent heaters are significantly more $$ than regular natural draft gas heaters (which require metal vent pipes that go more or less UP only). Some of these direct-vent heaters now require a fresh air supply from outside because they have sealed burners, you may not be equipped for that. You would need to be 100% sure of the condition, length, and route of the existing vent if you use it. Then there is the piping for water and gas and such, labor...

It might not be worth the trouble and expense to change back. Amortize the cost of the new heater over time and I am not sure it would pay off.