You have several ways to level.

First is of course to gather stuff. This can be sped up with survival manuals and cheap food. This also gives you stuff to sell.

GC turnins, gather the 10 HQ, sell all the NQ. Preferably in 10s for those who buy rather than gather for the GC turnins.

Then there are levequests, they are a fairly straightforward time for EXPs exchange. They give you nothing to sell.

Collectables are a hybrid of sorts. You burn your GPs gathering them, then go gather something else that will sell while your GPs refill and the next collectable pops.

I prefer selling things so I rarely do levequests, and hardly gathered collectables after I bought my folklore books.

Try them all, do the ones that you find most rewarding.