Black Bread  

There's a mithra called "Qhum Knaidjn" wearing the cooking outfit outside the Cooking Guild in Windurst Waters.

Talk to her. If she says you are too low of a rank to trade in items for guild points, then you need to become a higher rank in cooking.

Otherwise, the item to turn in for guild points changes roughly once every (real life) 24 hours. There is a limit on the amount of guild points you can get for each item (i.e., you can turn in about 5-6 stacks of Black Bread to get 2000 gp, but no more than that).

They will also take HQ (high quality or +1) synths of the item for that day, which are usu worth a few more GP.

It's best to lookup the item first, because sometimes you can buy it from an NPC much cheaper than crafting it. (I.E., you can buy Black Bread from a few diff NPC's in San D'oria for 110-124 gil each.)

Depending on what "rank" you are in cooking (craftsman, etc), the item you have to turn in is different. For example, someone who has 43 cooking has to turn in a different item than someone who is 56 cooking.

Talk to the Mithra standing outside the cooking guild to be sure which item you have to turn in.

Once you've built up guild points, you can use them to buy things like crystals that, when used, sign the item you synth'd with them (as long as the synth's yield is 1x; if it's 4x, for example, it won't work). You can also buy key items that help you cook, for example "Raw Fish Handling" costs 30k GP, and lets you make sushi.

GP is also how you buy the funky chef hat and apron ^.^

Hope this helps =p.

Same here on Garuda.