How much "output" stat do you need to get additional gathering chances?

[Catapult]

Hamster, you're an adder - I recommend picking up a Gridanian Doublet from the Order. The +20 Output will generally do it for you, but feel free to tweak with other things like gloves for a few more points to get the extra attempt if that doesn't do the trick.

This isn't tested, but I've got the feeling that it's around 25-ish in output.

But maximize Gathering before you maximize Perception once you start mining in Grade 5 areas - chance of success isn't something you want to neglect.

[Reonlasbard]

You're advising hamster to get a gridanian doublet, something I'd like to get as well. Are you supposed to have the job up to the level requirement of the item before you can buy it with seals?

[Conradus]

You're advising hamster to get a gridanian doublet, something I'd like to get as well. Are you supposed to have the job up to the level requirement of the item before you can buy it with seals?

The GC store will sell you anything you want if you have the seals, have the required GC rank, and, if it's Unique, you don't already have one in your inventory. Whether or not you can use it is your worry. You can buy it whenever, but you don't have one of the six classes it requires at the required level of 25, you won't be able to equip it.