Antifreeze in transmission 2010 Duramax??

You have a leak in the transmission cooler that is inside your radiator. And yes it's very possible it has ruined the clutches in your transmission. You definitely need to get the radiator replaced and the transmission flushed multiple times. Then cross your fingers and hope that the transmission survived.
bottom line get the radiator fixed right now.

^What he said.
And also a real kick in the nuts, as this is not a common issue. And not one to be expected nor prepare for. Especially in a vehicle that new ( unless it has a pile of miles on it maybe).

X3 as it softens the clutch’s friction material.

Well that sucks. :( Sounds like my luck.
In any event the glue that the tranny friction material is glued on with is water or antifreeze soluble. The friction material will be fine. The glue that holds it to the steels......not so much.
Don't ask me how I know. But mine was a cheap T350 I learned this lesson on and not a big buck Allie.
Good luck.

When I was in school I worked at a full service filling station part-time. Customer came in and wanted a quart of antifreeze added. I caught one of my co-workers just before he added it to the crankcase oil. After that he wasn't allowed to raise a hood. He did learn how to clean the Grease bay, restrooms, and wash bay.

bgum wrote:
When I was in school I worked at a full service filling station part-time. Customer came in and wanted a quart of antifreeze added. I caught one of my co-workers just before he added it to the crankcase oil. After that he wasn't allowed to raise a hood. He did learn how to clean the Grease bay, restrooms, and wash bay.

A friend I had in HS put 3 gallons of gas in a fresh water tank of a motorhome. That was not good!

ivbinconned, don't give up just yet. The issue of a failure of a transmission cooler in the radiator is a common problem for GM. The real issue how long did it go before you discovered the problem. In many cases you can simply replace the radiator, have the transmission flushed and the fluid replaced and you may be good to go. This is something you may want to consider before you just give up and purchase a transmission rebuild. Ultimately if the transmission has been toasted you can have a GM dealer replace the trans with a factory rebuild and you will get at 3 year 100k warranty to boot. Do your homework before you give up. You have lots of options!

Thanks ZR1
If my memory serves me correctly some older other GM vehicle I had a mechanic recommended disconnecting the Tran cooler lines at the radiator because of this kind of risk and we connected the oil lines to an aftermarket cooler installed in front of the rad.
Is this the right thing to do with this truck?

If your problem is the radiator, replace the whole radiator. You can then install an external cooler if you are worried about this happening again.

How much contamination did you find?

I found strawberry milkshake in my coolant bottle one time. Trans fluid looks ok but since I replaced the radiator just went ahead and did the trans fluid at the same time (I just use the cooler-hose into a bucket method anyways), and it's been fine ever since. I think that happened a bit before 300,000 miles and I have 502,000 miles on it now, and the transmission still hasn't been touched other than a couple more fluid services.