Gamorrean guards are some of the strongest melee troops in the entire Star Wars universe. They were employed by criminals and rulers such as Jabba the Hutt that were concerned with their personal safety.
They were seen throughout the whole galaxy in the position of bodyguards. They were extremely strong, and were thus talented at being guards. However being strong was one of their only strengths. While they were decent in combat, they were exceedingly dumb.
This was seen in the return of the Jedi when they were easily mind tricked by Luke. They were often described as being weak-minded as well.
Who knows maybe that was only Jabba's guards, but it sure seems like they are just dumb in general. They were actually pretty skilled in combat, due to their size and sheer strength.
Gamorreans were also seen across the universe fighting in many wars and skirmishes. They would fight as bounty hunters, mercenaries or guards. Their whole culture was about strength and combat prowess, so it should come as no surprise that they were better melee fighters than the average Joe.
They usually carried around Vibro-axes which complimented their fighting ability really well, because they could put a lot of power behind each blow. They would only ever use melee weapons because their society considered it dishonorable and cowardly to use any type of blaster.