according to your lodestone history you have not completed All Bark and No Bite, nor Unalienable Rights. these are two gladiator class quests that must be completed in order to unlock the ability to buy certain items from the guild store.

its the same with every other class as well, when you hit 20,30,36 bebop back to the guild and run the avalible quests. your pocket will thank you.

edit. well apparently i type way to slowly or took to long looking around.