You can buy i330 gear with Centurio Seals, as well as the items to upgrade them to i340. Because you can't get two of the same ring, you can only reach i338 that way, but any 350+ drop (or 340+ ring) from any content will get you to 340, which opens up everything except the most recent Omega raids and Four Lords fight, and savage. You can get enough seals (about 1,000) by doing your weeklies and all your dailies in both Heavensward and Stormblood for a few days (four at most, three if you get a weekly reset in the middle). Aside from buying gear, this is the easiest and fastest way to unlock Expert and all the other basics that let you get the actually good gear.