I asked a Japanese English teacher if I could use 医者がた. His response was no. His understanding of the use of がた was that it tends to be used for groups of people who are generally expected to be older than a particular group of people. For example, teachers are generally thought of as older than students so you can use 先生がた. Senior students are generally older than junior students so you can use 先輩がた. Parents are older than children so you can use 親がた. あなたがた is commonly used although it doesn't fit this rule. However, he said that there is no guarantee that a doctor or dentist will be older than their patient so he said they wouldn't use this construction for these groups of people. Any thoughts anyone?