The following website gives an e-mail address for the Maria Alm tourust offuce. Presumably, they can answer your question.
As you found in Bad Gastein - many things close in May in austrian resort areas. - this vacation time for owners and staff - this a very slow time as winter is finished - summer tourism has not begun.
Also a time for repairs and renovation.
You can find enough open that you do not starve or go without lodging but many fewer choices.
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