Head gasket

Absolutely. When there is no external leak, the cause is usually a leaking cylinder head gasket. There is a number of tests for that but you have described a real small leak. These tests might not work for such a small leak. The first one is a chemical test at the radiator. GM's engineers left the cap off most of their radiators, so you have to try to do this test at the reservoir. It involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If coolant is being lost through the head gasket, combustion gases are usually also getting into the cooling system. That will cause the fluid to turn bright yellow.

A better test when the leak is so small is to add a small bottle of dark purple dye to the coolant, then search a day or two later with a black light. The dye will show up as a bright yellow stain. If the head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the dye for the fluid you want to test, and those that rent or borrow tools should have a black light.

Neither job requires removing the engine unless there is some inaccessible bolt or bracket. Even that is very uncommon.

I do not think you need to remove the engine to do the job but you do need to remove the cylinder heads.

Here is a diagram that shows the heads off when doing the job (below):

Please let us know what happens.

Cheers, Ken

You should install new head bolts, spark plugs and have the heads resurfaced or the new head gaskets are more prone to leak. You should change the oil and filter as well. Here are some more diagrams to help you get the job done.

Install the cylinder head bolts.

Tighten the bolts to 60 Nm (44 ft. Lbs.) In the sequence shown.
Use the J36660-A in order to tighten the bolts an additional 95 degrees.

Let us know what happens and please upload pictures or videos of the problem.

Cheers, Ken