Can head gaskets be replaced other than replacing engine

Absolutely, but that type of failure is not as common as you might think. The biggest pressure difference is between the conbustion chamber and the cooling system, which are also the closest to each other, so the most common failure has combustion gases going into the cooling system, usually also leading to overheating, and coolant getting into the combustion chamber, causing the white smoke from the tail pipe.

The oil is under roughly 40 psi in the supply passages and 0 psi in the drainback passages. Coolant is held to 15 psi. Those three are all close to the same, so there isn't much force trying to push through the gasket. When it does, that's when you get the oil and coolant mixing.

The chocolate brown oil is pretty conclusive, but when you only see that in the coolant, the better suspect is a corroded transmission cooler inside the radiator's tank is leaking, allowing transmission fluid and coolant to mix. In either case, antifreeze in the engine or transmission will melt the soft outer layer of the bearings and bushings, leading to a catastrophic failure. It's possible that is what your mechanic is worried about. That damage takes some time to occur, so if this problem just started within the last few days, I'd take a chance on the head gaskets. Also, pretroleum products such as oil and transmission fluid that get into the cooling system with rot the rubber hoses from the inside. That won't show up as a leak for a while, but if it does, that is not a serious repair.

Also be aware some engines use an oil cooler that is a thick disc that sits between the oil filter and its mount. Coolant is circulated through that disc to remove some heat from the oil. Those discs can corrode through causing the mixing of coolant and oil. This will not cause the white smoke, but it is worth mentioning.

GM has also had a lot of trouble with leaking intake manifold gaskets. Coolant can leak out of the engine and be difficult to identify the location of that leak, and coolant can leak into the intake system and get sucked into the cylinders. There's your white smoke. There's usually no oil passages in the intake manifold, but if coolant leaks out the bottom, it will drip into where the oil is dripped and sprayed onto the camshaft lobes. In this case there would not be oil getting into the cooling system so the coolant would look okay except the level would keep dropping.

The last thing to be aware of is GM is still using the miserable red "Dex-Cool" antifreeze. The first insult is they advertised that as "lifetime" antifreeze to imply their cost or regular maintenance was lower than that of their competitors, but then they put a sticker under the hood that said to replace it every three years. Even the Dex-Cool company doesn't recommend that. They say to replace it every two years, just like every other brand, because that is how long it takes for all the critical additives to wear out. A lot of mechanics call this "Dex-Mud" because of what it looks like when it gets ignored for too long, and when it is mixed with other brands of antifreeze. When you only see that mud in the cooling system and not also in the oil, a flush and refill of the cooling system is in order.

It really matters how much coolant was mixed with the oil and how long it was driven that way. Is there any knocking or noise from the engine? If I provide the directions, would you like to do it yourself?

Let me know.
Joe

It's still a matter of time. Engine bearings and transmission bushings are made of three layers of metal. The first layer the oil touches is very soft so particles of harmful dirt will be embedded in them rather than scratch the highly-polished crankshaft journals. It's that soft outer layer that gets melted by antifreeze. Water alone lowers the oil's ability to isolate moving parts, but it won't cause that damage to the bearings, so if only water has been getting into the oil, there is less likely to be that kind of potential damage.

The color you described suggests there is antifreeze in the oil, and if that has been happening for, ... Oh, ... Say, a week, an oil change right after replacing the head gaskets, then another one a few days later might save the engine.

JACOBANDNICKOLAS, please add your opinion as to time, likely hood the diagnosis is correct, and the chance of a successful repair.

I would agree with the above, if it has been about a week or less and the driving was short distances you might be able to save the engine. However it's running on borrowed time.
At 140,000 if it were mine I likely would opt to replace the engine as long as the rest of the truck is good.

Being a 6.0 you can do either. They are a common engine. A long block runs about $3,000.00 or so depending on where you get it from. But shop around your area. There are multiple companies that build them.