For the record, I don't believe Angel will successfully resurrect Giles. Could be for any number of reasons. Angel realizes it's the wrong thing to do. He can't find all the pieces of Giles' soul. He finds the piece that won't stop telling him this is wrong until he gives up. Or that piece keeps yelling until Faith thinks he's gone mad and stakes him. Or that Egyptian soul thing gets ripped out. Or Faith finally honors Giles' wishes, puts her foot down and stops this nonsense.
That being said, I'm not sure what will happen if Angel does manage it. That Giles will be furious is a certainty, but what twill he be able to do about it. I was reading some Australia-era X-Men comics I picked up recently, but I don't see it being similar to what developed between Rogue and Carol Danvers, where Carol could take control at times. If that happens here, there's a good chance Giles will walk them out into the sunshine. Which wouldn't bother me too much (I'm cool with it as a solo book for Faith), but they probably don't want to kill the character that gets top billing.
I suppose Giles could end up being a helpful presence eventually. He starts of angry, walls himself of in a portion of Angel's mind, but can't help from offering biting commentary, and then, the occasional useful piece of information. Eventually, he starts helping more regularly, and Gage could play with the idea of whether Giles has really come around, or if he's biding time until he can set them up. Has he come to grips with his situation, or is he still desperate to escape? If it's the latter, how far is he willing to go? Get Angel dusted, get Faith killed, destroy the whole world? Hmm, Giles could in essence become another Angelus. Someone Angel keeps locked up in his head, buried under guilt, who goes apocalyptic on things the second he gets a chance*.
The trick is - and this is something Angel has to cope with as well - we don't know how this will work. All are Giles' traits and knowledge going to be absorbed by Angel, like adding a bit of one color of paint to another. Or are they going to be two distinct entities, sharing the same body. Who has control, can it shift, if Giles feels perfect happiness, will Angel lose his soul, we have no idea.
* I can't recall where I read it first, but someone pointed out years ago that Season 2 Angelus didn't bear much resemblance to pre-curse Angelus. The one who ran with Dru, Darla, and Spike didn't have any interest in throwing the world into hell. In his own way, like Spike, he liked the world. Too many amusements and nice things to destroy it. But after 100 years of rats for food, the brooding, the terrible hairstyles, being blamed for every bad thing they did, when we've seen that Liam was a miserable waste of blood alive, and Angel's no choir boy, Angelus was just too angry. He wanted revenge on everything.