"GIN, the most know non-default index type perhaps, has been actually around for ages (full-text search) and in short is perfect for indexing columns where there are lot of repeating values – think all kinds of statuses or good old Mr/Mrs/Miss. GIN only stores every unique column value only once as for the default B-tree you’ll have e.g. 1 millon leaf nodes with the integer “1” in it."
Does it mean that GIN is a very good choice for low cardinality columns.
Not necessary. There is other index which also don’t keep column value in an every leaf. Hash, for instance.
