Consume data from other database motor from stored procedure in Oracle 12c
Database Gateway and Generic Connectivity (DG4ODBC) Licensing Considerations (Doc ID 232482.1)


  The Oracle Database Gateway for ODBC (DG4ODBC) is included in your
  Oracle Database Server license without extra cost. 
  
  ... 
  
  While the
  Database Gateway for ODBC (DG4ODBC) does not require an additional
  license from Oracle, a third party ODBC driver is required, which may
  require a purchase independent of Oracle.


Yes, Database Gateway for Informix requires the purchase of additional license.

No, Database Gateway for ODBC does not require the purchase of additional license.

All you need is a 3rd party ODBC driver which is compatible with Informix, then you can configure DG4ODBC to use that.

I have done this with IBM DB2, PostgreSQL, SQL Server before, it should work with Informix as well.