While leveling GP is useful, but not necessary. Most of the time you won't have it (because it replenishes so slowly), but it's very nice for the nodes where you can use it. HQ items yield much more exp than NQ and sometimes you'll want to increase your gathering chance (particularly on items you've never gathered before since those will cap at 25% until gathered the first time). Keeping jewelry up to date (and/ or HQ) is more a luxury than anything while leveling. Like others have said 600 is necessary (more or less) at 60 to do any endgame gathering, while 650+ is very nice to have.

As a side note, if using a DoL retainer they never need any jewelry (or any purely perception pieces really) except to meld gathering materia into it if desired/ required. All of their ventures are based on the gathering stat.