When you're level synced, your stats and your gear sync down to an appropriate level, too. You'll end up with roughly average performance for the level you're synced to. If it's anything like FFXI, wearing HQ gear will give you better synced stats than NQ gear, but not as good as if you wore HQ gear that's actually for the level you're synced to. In other words, syncing to level 10, a level 10 HQ chest piece > a synced level 20 HQ chest piece > an NQ chest piece synced or otherwise.

Busere, level sync is a system that temporarily lowers your level to the maximum appropriate level for some bit of content (a quest instance, a guildleve, a guildhest, a dungeon, a FATE, etc.). Your level and your stats, along with the stats on the gear you're wearing, get lowered to what they'd be at that level until you're done with the thing you're syncing for, then you go back to your natural level. For example, I'm level 39. I can still run the first dungeon, and when I do, I'm effectively level 17 until I'm done with it. I still earn XP (the same amount I would at level 17, so not enough to make this an effective leveling strategy at level 39, but it means I can help out lower-level FC mates etc.).