I had my truck in for service - 2006 Sierra Denali - due to an oil leak. They replaced the "engine oil cooler lines to pan gasket". Within a couple of days had a major issue - coolant had gotten into my engine. This seems too coincidental - could the problem have been caused by the part replacement being installed incorrectly?
This should be what your engine oil cooler lines look like in the vehicle:
If the oil leak was coming from the block (middle of the vehicle), part #1 should be what they changed to stop the leak. This would be the most likely place for a leak to occur in this system. The lines going forward should carry oil to the radiator, where heat is exchanged. In order for their to get coolant into the engine from anywhere in the cooler loop, it would have to come from the radiator. If this is the case, the shop which worked on your vehicle should not have even been touching the forward area at the radiator to fix the leak. Realistically, if they just replaced the "gasket" as you said, there should be no real reason for their work to have caused the issue.