You should focus on getting to 99 above all else. Assuming you level in Abyssea, there is not much point in investing in gear on the way to 99 (aside from weapon) since you'll be selling it back almost immediately. From there you have all sorts of things you can do, although farming seals and +2 items for Lancer set is probably the best use of your time when you hit 99. There are also other random Abyssea things you'll want, like Atheling Mantle, Goading Belt, Twilight set, Nierenschutz. Also make a Chauve-Souris, it's a nice polearm (from Trial of the Magians). Dynamis farming is not bad on DRG even solo, and 4/5 of the relic +2 pieces are useful in some way. Eventually you'll want to look at doing Neo-Nyzul, as you'll be able to make use of all 5 Phorcys pieces. There are some nice Ogier pieces from Voidwatch that you'll want for your -damage taken sets (also Ganesha's Mala and Mekira Meikogai). Meebles is the other newish endgame event but you'll need a static for that really. Kokou Earring and Letalis Mantle come from that. Enif Corazza comes from Arch-Omega and there probably some Ares +1 pieces from Neo-Salvage that you will want. There are also many AH options for things like Mekira-oto, Brego Gloves, Justicier's Torque, Prosilio Belt, Rancor Mantle that are best in slot as well.
Okay so I basically went through my gear and went over where it comes from. It's not very organized but as you can see, there are many events that offer useful pieces (often best-in-slot) for DRG, but you'll need to be level 99 to participate in the events that drop them. Keep in mind that right now DRG isn't in a particularly good place in the hierarchy of DDs and for some things you may get passed over. But if you gear intelligently and play aggressively, you'll always be able to hold your own.
Philemon on Valefor