Bringing Gillian back, giving her some measure of redemption, then punishing her for her past crimes wasn't a bad idea. The problem was that her entire story this season was unrelated to everything else on the show. At least in past seasons she was tied up with Jimmy or Rosetti, but this year she was doing her own thing and they unnecessarily dragged that across the whole season.
Meanwhile, there's a bunch of interesting storylines that have developed into a bit of a mess. The finale has to resolve the Dr Narcisse story, the Eli/Bureau storyline, the Chicago stuff, the Chalky/Nucky stuff, potentially some stuff involving Rothstein, and some stuff involving Richard (according to the trailer). That's a lot of material, and I fear that it will be rushed, just like the season 3 finale. All that time spent on Gillian could have been better spent spacing out the other stories so we didn't get this end-rush.