It's not so important to keep your gear equal to your level, but rather keep it equal to the current quest level.

Dungeons will synch you down to the level of the quest that unlocked them, so as long as you're geared appropriately for that lower level, it won't be an issue that your gear doesn't match your real level!

For earning GC seals, a good way at this point is doing the daily "supply missions" that request crafted and gathered items (one per class) and reward you with seals. You also get them from the leveling and guildhest roulettes (with a good bonus for being the role currently in need), FATEs, GC levequests, and once you reach a certain rank you'll be able to trade in the aetherial gear you get from dungeons.

Also, a word of warning if you're using a tank class (gladiator/marauder): DON'T use the "recommended gear" button for these classes without double-checking that the recommended upgrades are actually a defence boost! It judges based on the vitality stat, and can get confused by higher-level-but-lower-defence gear intended for other roles. This is only an issue up to Lv50 where the gear system changes to have more rigid categories for each role, and mage gear stops being marked as equippable by all classes.