I'm assuming you're not 50, if you're mentioning leves.
Leves are definitely a good way to level up, but you'll blow through them fast. I generally made an HQ gathering set every 10 levels. This helped ensure that I'd always have a 99-100% gathering success rate on the default targetable items from the leves. (A lot of leves also include an "HQ" item you can gather for more points at a lower % success rate).
Not all leves require you to blow your GP. A leve that asks you to gather 40 items, and gives you 15 minutes to do it, doesn't really warrant you spending your GP to increase your successrate from 89% to 99%.
Most leves require you interact with 6 or 7 nodes. You get 5 GP per swing (I think), and you get GP when running around between the nodes. You also have a rediculous amount of time to complete the leve and still get the "quick completion" bonus. So with 500 GP, there's no reason why you can't use your 100 GP +10% gathering success rate ability on 6+ nodes per leve.
So as a general rule, I'd:
- Grab my two leves
- Do one that would require me to use GP
- If the other leve is the gather 40 items from unlimited nodes, immediately do it. If it's another leve where I'd want to spend GP, then whack away at nearby nodes for exp, items and GP until you're full on GP, then do the 2nd leve, using GP.
- Turn in the two leves, grab two more
- If I have full GP, use a leve that needs me to spend GP. If I don't have full GP, but I have the gather 40 items leve, do that one to fill up my GP, while fulfilling the leve. If I don't have GP, and am missing the 40 item leve, whack at nearby nodes for GP.
- Repeat
There's a leve at each location that you can get a 25% bonus from by having a "high score". And it tells you the targets to aim for in the leve progress bar. On those ones, it's always nice to have top-notch gear to help make it easy for you to hit those harder-to-target items.
And, like regular leves, always increase the difficulty. Or rather, the system will automatically increase the difficulty for you. I didn't do a level 25 leve, as a 25, and increase the difficulty by +4, or anything.
Basically I just made sure I was always gathering something, and placed a huge emphasis on inserting leves into the rotation without just waiting around for GP to replenish before doing the next one. The leve completion bonuses are too huge to ignore. Of course, you could always just walk back and forth in between 3 nodes for items/exp. But it won't be as fast.