GST stands for Goods and Services Tax.
Goods mean tangible things such as equipment, furniture and clothing etc.
Services mean intangible things such as accounting services, legal services, dry cleaning services and employment services. The term anything else can include such things as an obligation to do something or not to do something, agreements, options etc. The short answer is that GST is based on supplies. The term supply is explained later but, in the GST law, it is used in a much broader sense than simply selling something. Supply does not have to involve money, nor does it have to be legal.