This was recently added to CREATE INDEX reference page:
+ The optional <literal>USING</literal> clause specifies an index + type as described in <xref linkend="indexes-types"/>. If not + specified, a default index type will be used based on the + data types of the columns.
But I think this is wrong, the default type is BTREE, it does not depend on the data type. Or at least, I've never witnessed the claimed behavior. The claim also conflicts with what is said at https://www.postgresql.org/docs/10/indexes-types.html