So I've been rank 20+ armorer for about 5 leve resets, and I can't seem to figure out how to earn these marks. Are they randomly given on local leves that are rank 20+? Are they not given out locally? I just can't seem to figure this out.
They're given out, seemingly at random, in place of the gil component of the reward. You have no way of knowing if you're going to get guild marks or not until you actually turn in the leve and see first hand.
There's a theory that I've been trying to test that suggests finishing the required items with a final assessment greater than 50 increases your chances of earning guild marks instead of gil. Sadly, and you'll forgive my broken record kvetch about the issue, but crafting above about rank 15 is broken as **** right now and getting anywhere near 50 on a rank 20 leve (much less 25) as a rank 25 armorer has been next to impossible. I'm lucky to complete the leves at all.