what does it mean if the head gasket is leaking ??

The seal between the combustion chambers and water jacket(s) and/or oil passages is blown out, you may have coolant in your motor oil, be blowing smoke out exhaust, other things. Bottom line, it's serious and expensive

First pull out the dip stick and check the color of the oil in the motor. If it is a cream color, you're getting water into the motor oil. That is a positive sign you may have a head gasket leak. Are you losing cooling fluid? White smoke may be coming out the tail pipe as the water is turned to steam and blown out the tail pipe. When the car is cold, before starting it, remove the radiator cap, check the fluid level. Leave the cap off and start the truck. If there are bubbles passing by in the radiator fluid, it may indicate a leaky head gasket as it pushes air into the cooling water jackets as it goes past the heads. Have you noticed a loss pf power? You may have one or more cylinder losing power due to loss of compression. If you have time, take the truck to a mechanic and they can test the cylinders using a compression tester to find which cylinders are bad. Another little more advanced method is to look at the spark plugs and see if they are carboned up from poor ignition due to coolant being in the cylinders. Several choices, just replace the gaskets, or do a completer head job. Or depending on what is found, an engine rebuild may be in order. A good honest fair mechanic can guide you in the right direction. Make sure they can show you and explain to you the work they recommend. At times when a head job is done, it will put more pressure on old pistons and rings that will cause them to fail and start using oil. The smart thing, is when the heads are done, do the rings also. That will mean a lower end rebuild. So be aware, depending on what is found, it can be a few hundred dollars work, or a grand and a half or a little bit more.