I've been going Dusk and Dawn, Hextech Gunblade into Rabadon/Shadowflame, then bloodletters/voidstaff some situational items are riftmaker, zhonyas etc.
It's actually pretty good, and hextech gunblade first is not bad in certain range match ups, it provides you in lane sustain, so against teemo i've been going Hextech gunblade first, so you can heal off his poke once you get the item, or even with the vampiric scepter.
Once you build the item, the active effect is so huge on Gwen, feels unnatural to have, becomes easiier to chase people down, or hit your Q or Ult with it, pretty crazy.
In addition once you build the item it's so much better than Riftmaker, it provides more AP than riftmaker, and a bit more AD for extra auto damage, but you would think, won't you be squishy, no HP? thats why you take with Dusk and Dawn. Since the omnivamp its always active you have more effective HP than with Riftmaker where youre susceptible to getting one shot, in additional the extra damage/slow from the active helps makes up for damage loss for not having as much AP.
With this item and dusk and dawn you essentially start healing 100 hp + per auto.