How old is the chick? Is there any chance of injury? Can it walk or stand? The gasping could be pain related or it could be something respiratory. Have there been any other symptoms other than the lethargy? Any wheezing, sneezing, runny nose or diarrhea? Any other chicks with issues? The dust bathing shouldn't have anything at all to do with this. Have any other members of you flock ever had respiratory issues that they have recovered from (and possibly be carriers now)? I wouldn't put the chick back under the momma if you have an alternate heat source. The momma will care for the active birds but probably won't nurse a sick or injured chick back to health. The whole survival of the fittest thing.