How critical is it to use only dex-cool in 2001 blazer can damage to manifold happen if this is not used?

I had the car into be serviced in october and they topped off the collant with the other stuff - but my owner manual reallly states to use dex-cool Only. This week I had a problem with it over heating and now the same garage says I need to spend $250 on a manifold gasket repair job.