Being kicked from dungeons, not able to complete quest

[Thaliss]

Maybe just switch to a dps role for starters. And if you do don't choose Marauder (axe guy) it is another tank

To summarize game is around trinity :

Tank (takes damage, hold aggro/enmity on all ennemies) - Blue icon in party list
Healer (keeps team alive) - Green icon in party list
DPS (classes meant mainly for damage and support) - Red icon in party list

In dungeons you get 1 tank, 1 healer, 2 dps

But you could stick with your gladiator.
1/ try to have your equipment up to date with your level, means spending some gil to buy some on the market board
2/ look carefully at explanation of your skills, specially those marked "generate additional enmity", and those that are part of combo (if you do skill A first then when you do Skill B it will have additional effects)
3/ at 1st dungeons try this when you are pulling a group of enemies :
- Target one and use Shield Lob
- Run to the group and when they are near you use Flash Twice
- Try to place yourself so you are facing all enemies because you don't block or parry from behind
- Start using combos Fast Blade then Savage Blade
- Switch between enemies and land combos on them
- use flash from time to time
- use your CDs, at your level it is Rampart, Fight or Flight, Convalescence

This enough for the first dungeons.