what kind of damage will water in your transmission couse

Mostly corrosion if left in there , you can drop the pan but you never get all the fluid that's in the converter , but then I hear a total purge may not be good for it either even as a maintenance thing . I'd drop the pan and see what comes out . I've removed the coolant lines to the radiator , put one in fresh fluid and the out put in a waste pan with no ill effects that I remember . The hoses attached to the line should be secured .

Worse case, bye bye transmission. I would drop the transmission pan and see what comes out, of course this being a Silverado that’s easier said than done, but you need to do it, or have it towed to a good transmission shop and have them do the dirty work and check it over.