A Letter of Authorization for GST is an important document to share power. How to Craft a Letter of Authorisation for GST Registration?To obtain advance information from recorded highlights of selected documents to appear in the next issue, dial 2025235022. A weekly newsletter from Newton MA Ward 5 City Council candidate Bill Humphrey. Government Printing Office, Washington,. In a firm, the person responsible for managing the GST proceedings has to often fill the forms themselves or pay a visit to the GST office. This application may contain personal information as defined under the Municipal Freedom of Information and Protection of Privacy Act.