If a diesel truck has been run dry of coolant and has then been refilled and then overheats and now has a blown head gasket when was the damage done

I would recommend a proper engine evaluation to ensure you're made aware of all possible damage however, most likely cause is warped head/s. Of course, the head/s could also be cracked in which case you're up for a decent amount of cash to fix.

Not sure if you have the know how or tools but just remove the head and send it in for assessment. If warped, they can resurface the head/s, reseat the valves etc. and hopefully its nothing more serious than that. Then all you need is a new gasket, new head bolts and chuck it back in. (yes there is more to it but these are the broad brush strokes).