Hi Luis,
Once you log in to CA Support Online (https://support.ca.com), you should be able to go to "Download Center" -> "Download Products". Search for "CA Test Data Manager" and enter the release number you are interested in (ideally the latest GA as of today, 4.0). Click "Go" and you should be able to see two files available for download. One is the TDM product full package, and the other is the repository installation kit (CA Test Data Manager Repository Kit 3.2.6, for example with TDM 4.0).
After you extract the repository install kit to your local drive, you should find gtrep.bak under the "DB-install-kit-3.2.6\SQL_Server_Install_Kit\Databases\DM_Repository\" folder.
For instructions specific to installing a MSSQL repository, refer to this link: Install the Repository - CA Test Data Manager - 4.0 - CA Technologies Documentation
Hope this helps.
Thanks,
Sameer