i have a 1992 gmc jimmy

It's very likely that the head gasket is your problem. These gaskets often leak if the engine has been overheated, but can occur on an engine with high mileage too. The next step is to have a shop remove the head(s) and inspect the head gaskets.

probablly the head gasket, without havein g to replace it, and you still get good power, put some Lucas motor honey, or Stop Smoke in. should stop smoking between oil changes.

the problem may also be your valve guides

I also have a 92. It blows out white smoke after I start it up for about 5 mins. Had everything checked and nothing is wrong. So really dont have an answer on it.

If it is a 4X4 you need to check the vacume valve on top of the transfer case. They will cause transmission fluid to leak into the vacume system and cause a white smoke problem. It will also cause some of the actuators on such things as the AC/Heater vent/doors to fail.