Gmc head gasket??

All of them make moisture out the tail pipe as moisture is given off during the combustion process. A down and dirty redneck method of checking the tailpipe water for coolant is put a small drop on the tip of your finger and touch it to your tongue. If it tastes sweet it is coolant, if it tastes like nothing it is water. If it is coolant a head gasket or cracked cylinder head may be the problem. Also coolant will usually produce a thick white smoke at the exhaust. Warning do not ingest antifreeze it is poisonous, but a small dab on a finger won't hurt you. Also have the trouble code read at a local autoparts store. This is usually a free service, just don't buy anything based on their recommendation.