The calling of stored procedures is highly dependent on the driver and the database. Note that retrieving result on PostgreSQL database is very similar to querying tables. The Query or QueryRow method of the DB struct pointer is called, and the resulted rows or row pointer can be parsed to obtain the values.
The general problem for almost all drivers ia the OUTPUT parameters of stored procedures. Go 1.9 added support for such parameters, but the majority of drivers of commonly-used databases do not implement this yet. The solution, then, could be the usage of the driver with a nonstandard API.