The company's logo.
Gruppe Sechs is a security company that is featured in Grand Theft Auto III, Grand Theft Auto: Vice City, Grand Theft Auto: Vice City Stories, Grand Theft Auto: Liberty City Stories, and Grand Theft Auto IV. The company primary vehicle is a Securicar, which they use to transport goods and money. The company doesn't appear in Grand Theft Auto: San Andreas, as it was replaced by another security comany called Chuff Security Co. and Lock and Load Security, which the logos are featured on the Secricar instead
The name "Goupe Sechs" is German for "Group Sex" as the name sounds like this when being said. It is also a pun of the infamous European security company Group 4. In GTA IV, the written logo is changed to "Gruppe 6" which still translates to "Gruppe Sechs" in German.
In GTA Vice City and GTA Vice City Stories, the location of the company in the city is unknown. The company's cars can be seen driving around in Viceport. In the PC version of GTA Vice City, a Securicar can be found parked in front of the El Banco Corrupto Grande.