Microsoft is currently only providing an ODBC driver for MS SQL Server for MS Windows or Linux platforms. So one option you have is to move DG4ODBC from the Solaris platform to Windows or Linux and then connect from your Oracle database on Solaris to DG4ODBC installed on a different machine which will then connect with the ODBC driver to the SQL Server.
In addition there's also a free ODBC driver from FreeTDS which you can compile from a source file on Solaris. This driver requires (as many other ODBC drivers as well) an ODBC driver manager which is available from www.unixODBC.org also for free.
The disadvantage of FreeTDS is that it might cause some problems with DG4ODBC as DG4ODBC requires a level 3 compliant ODBC driver which fully supports the 64bit standard and when you now have an issue that is related to the ODBC driver you might have problems to get a fix from them.
So please take care if you really want to use a driver for free in a production environment instead of purchasing a commercial ODBC driver for example from DataDirect, Easysoft or Openlink.
Openlink is fine, but the driver you mentioned i wrong. You are trying to connect from your Oracle database so you will need an ODBC Driver for the SQL server. As you are going to use DG4ODBC on Solaris the driver must also be a 64bit driver.
There's no certification process of foreign ODBC drivers that can be used with DG4ODBC. The requirements are documented on the manual and best would be to install the driver and test it. Most vendors offer a trial period that can be extended for some time.
There's no document available how to configure DG4ODBC with FreeTDS, but you can use the note based on the DataDirect ODBC driver: How to Configure DG4ODBC on 64bit Unix OS (Linux, Solaris, AIX, HP-UX Itanium) to Connect to Non-Oracle Databases Post Install (Doc ID 561033.1)
Just make sure to use the correct libraries (unixODBC Driver manager library is commonly called libodbc.so) and verify the ODBC driver is working before you start configuring DG4ODBC. The commands I commonly use to compile unixODBC and FreeTDS are:
./configure --prefix=/u01/odbc64/product/unixODBC --sysconfdir=/u01/odbc64/product/unixODBC --enable-gui=no --enable-drivers=no
sudo make install >>make.log
./configure --prefix=/u01/odbc64/product/FreeTDS/0.91 --with-unixodbc=/u01/odbc64/product/unixODBC --enable-msdblib --with-tdsver=8.0
sudo make install >>make2.log
The odbc.ini file I use looks like:
Server = sqlserver.de.oracle.com
#Database = AdventureWorks
Database = gateway
Port = 1433
TDS_Version = 8.0