You should level other classes so you can slot additional skills from those classes regardless of what "job" you're going to shoot for. You get one additional skill slot for every 5 levels, every 10 levels if you have a job Soul equipped. There's a lot of discussion on the various recommended skills etc here and on other sites.

Once you complete the level 30 marauder class quest, IF you have level 15 GLD completed, you will trigger the Warrior Job quest.