Part of the problem here is the WoW-centric point of view where each class needs to have viability in a number of different areas. Instead of looking at the Gladiator class as a name, look at it (and other classes) as a concept instead and you can begin to understand how the system works with greater detail.

Gladiator - Defense and mitigation tank
Marauder - HP sponge tank and enmity generator with skills to disable.
Pugilist - Avoidance tank with good situational damage skills.
Archer - Ranged DD with high mobility and good enemy mobility impairment.
Lancer - Melee DD with great support abilities and party buffs.
Thaumaturge - Magic damage focusing on DoTs and utility debuffs.
Conjurer - Magic damage focusing on DD spells and party buffs.

Keep in mind that these are generalizations and every class has some abilities that break out of this mold.

However, the way to look at classes is to pick the generalization above that most appeals to what you want to do. That's the class you should play. Now pick 1 or 2 other generalizations that appeal to you, those are the secondary classes to level and will dictate the abilities that you want to add to your main class.

Want to be a great all around tank? Play gladiator and rank up marauder and pugilist to support.
Want to focus on amazingly powerful aoe magic spells with solid party utility and debuffs? Play conjurer and rank up lancer and thaumaturge to support.

Etc.

You will always be disappointed with a class if you are trying to make it do something outside its generalization. If you are trying to be a gladiator melee DD, you are not going to do well because that's the way the class system is designed.