Maybe we first need to set the parameters of circumstances, timeline and location etc. for them to fight? Willow in the strength after sucking the Magic Box dry or after Rack... or Giles?
Glory in her half-human form (greatest weakness is Ben) or pure Glory like she was in her hell dimension? What do they know about each other? Where do they fight and when?
If it's set like in a ring match (three, two, one... FIGHT!) where both go immediately at full force, Glory wins. She'd just rip DW's heart out or does other gruesome and deadly stuff, using her incredible speed and strength. *shudder*
If not, I need parameters. Glory didn't kill S05 Willow after all (she would have if it weren't for Buffy, but she was about to take her time). And Dark Willow could easily pull off measures that had lead to Glory's defeat in "The Gift". DW>S05 Willow; DW> Dagon's Sphere; DW >Buffybot; DW> Olaf's hammer; DW> wrecking ball and DW> S05 Giles. IMO.
Just suppose Giles never killed Ben, sparing Glory. When Dark Willow drains Giles, Glory appears for revenge in her weakened human form. Obviously Glory (from past experience) would know what Willow is capable off and based on Willow's appearance, would probably guess that she is more powerful. Glory could end the fight here if she uses her super speed to brainsuck or kill Willow. However, if she doesn't, Willow would go on the offensive, probably hitting her with lightning and fireballs. These would slow Glory down but not do any serious damage. If Dark Willow were to win, she just needs to hold Glory off until she reverts into Ben however that is easier said than done because the moment Glory uses super speed, the fight is over.
In a random encounter, Willow might stand a chance as Glory will probably underestimate her but I'd still give Glory the win. Glory at full power in her true form is unknown as we have never seen it however It's a pretty safe bet that Glory stomps. We are talking about a witch with 3-4 years experience against a full powered god that ruled a higher hell-plane of existence. However, Willow in the comics destroys Glory in her human form as she (Glory) hasn't appeared in the comics so her power hasn't been expanded on.
Individually each of those weapons in 'The Gift' is beaten by Dark Willow however if you use them all together continuously then I believe the opposite. Glory was bar-none, the hardest villain on either show (Btvs/Angel) to defeat. Her ultimate defeat took not one or two brilliantly inspired ideas (as implemented towards other Big Bads), but a total of eight: Olaf's hammer, the Dagon Sphere, Willow's brain suck, Tara's sense of direction, the Buffybot, Xander's wrecking ball, Giles' ultimate murder of Ben, and Buffy's ultimate sacrifice. If you hit Willow with the things that was used against Glory, she would get destroyed.
Remember Willow was actually defeated by Giles initially when he placed her within that binding spell and if she wasn't their friend, it is possible that he could have killed her.