Thank you. You and I have never discussed this before, and I know you like a full explanation. Gwen's death is a fan favorite topic in the comics forums, been discussed a hundred times over the years, so these things just stay with you lol. 
Peter is absolved of all blame in TASM 2 because Gwen went straight into known danger, and against Peter's wishes. Breaking Stacy's promise and not going to Oxford are not what killed her. Gwen would have been alive and well if she had simply done the sensible thing and not gone into the middle of a super villain battle. But she did, against Peter's wishes who tried to verbally and physically stop her, and as she spelled out it was her choice, not Peter's. So he is absolved of feeling any guilt for what happened to her.
Whereas in the comics, he never told her he was Spider-Man, so she had no idea she was in a dangerous relationship that could make her a target, and it did, and she died for it. There is none of this great tragedy there in the movie. Gwen has only herself to blame for her own death in the movie. Peter could easily have wept over her dead body saying "Why didn't you stay away like I told you. WHY WHY WHY?!?". In the comics there was nothing Gwen could have done to save herself because she had no idea of the potential danger she was in thanks to Peter.
Exactly 
		 
		
	 
Ok, so I'm going to try 2 different logical strains here, because I can get long winded when I try this, you're free to break down my post if you feel the need to, I was being a bit of a (nasty guy) when I told you it annoyed me, haha.
1)
Ok, so first thing I'm going for is a logical chain of events that lead up to Gwen's death, if an event was dependent on another event, the blame can be pushed back, if the event if irrelevant to the overall consequence, (Gwen's death) it is not counted. 
Going back to TASM;
Gwen meets Peter- No
Peter asks Gwen out- No (or POSSIBlLY yes, if you want to get REALLY philosophical)
Peter Kisses Gwen- No
Peter saves Gwen @ School- No
Captain Stacy dies, tells Peter to leave Gwen out of it- YES, this was an explicit directive to keep Gwen out of danger, regardless of his feelings. Peter has now made an explicit promise to keep Gwen safe. It is now his personal responsibility, to prevent action or inaction that contravenes this promise.
Peter "Those are the best time"- Peter breaks this in about 5 film minutes with perhaps the strangest, borderline pychopathic, lines in CBM history, it was cute they got back together though. This basically breaks the chain. We need a new link.
TASM2
Peter Chats to Gwen on Phone- He's breaking caps promise, but he's ignored him right?
Pete sees cap- This shows that Peter has a moral weight on his shoulders, thus reinstating the promise as the predominant moment. Whilst Pete's blame was previously on the "best kind" line, the blame will now preceed this moment.
Pete kisses Gwen- This shows that Pete can feel guilty about the promise, whilst still being in a relationship with Gwen; indicates the promise doesn't how much weight at this point.
Pete is dumped by Gwen- Chain reinstated, Peter says he can't be with her anymore, 

() Gwen gets mad and acts like Emma Stone for a bit and cuts him loose.
Pete stalks Gwen- Unneccesary to mention, but I just did.
Song for Zula Scene- Gwen initiates contact, under the proviso of "just friends", however, this kinda turns into a mini date? Gwen just intended to tell him she's buggering off to England, sooo, we'll leave it here. 
Closet Scene- Peter Kisses Gwen, promise broken. New point of blame
Oxford Scene- Gwen "Maybe we're on different plans" Attempt from Gwen to inadvertantley keep the promise, as such, promise is momentarily kept.
Peter sulks in roosevelt- Unrelated but he found a train! (these tid bits are to keep you entertained by the way)
Gwen Phone dumps pete- Definative solidification of the Oxford scene, promise is now in play. As it stands, the promise will be kept, and Gwen will not die. The promise is therefore becoming a central tenant of Guilt.
Bridge- Peter reinitiates contact, openly admits that no reason will stop him being with her. Final, break of the promise. This means accepting "1 million reasons" why they should not be together are a possibilty, but their love will trancend that. That means accepting things may go bad, that she MAY die (they have recognised this). 
Power Plant- Gwen fights against Peters wishes and stays at the powerplant. Promise is already broken, and Gwen could NOT have been at the powerplant without Peter's previous actions. As such, Peter is retroactively to blame for Gwen's actions.
Gwen dies- Yeah...
To conclude, Gwen died because Peter held a long time promise, but that's ok because those are the best kind 
 
In seriousness, I think that's my overall position as to why Peter is to blame for the Death of Gwen in TASM2. Feel free to refute it if you wish, I'm an odd fella for just taking a "chain of events" approach.
Now, let's look at the situation practically. (2) (Possibly more subjective I feel)
What is an easier action? 
To physically force Gwen out of the powerplant? Who is to say he cannot webswing her away from the place whilst electro is knocked out? 
To just not take her on the bridge?
Obviously action 2, but even if we say action 1, is it logically possible to say that Gwen overpowered a superhero?
Peter had the means to move her, he had the means to not even HAVE to move her, he didn't though. He could have not stood around like a stoned fish and actually pulled some spider sense off and swung gwen away as Goblin did his 12 second double take on the two.
Peter was at fault, in a similar sort of ambigious, not direct, way to the comics.(He didn't shoot gwen, he was around gwen and facilitated an evil act to occur through his prescene that led to Gwen's demise)
I feel that's at least 50% logically sound and 50% rambling. Close enough