If you want to answer the question literally, it would be George (Myrtle's husband), since he pulled the trigger of the gun that shot Gatsby to death (and then shot himself).
However, let's back up a few steps and follow the chain of events that led up to this act.
Backing up one step, we find George talking with Tom who told George that it was Gatsby's car that ran her over (even though Tom knew full well that Daisy was driving it at the time of the accident). So you could rightfully say that Tom was responsible.
But, back that up a step and we see Daisy who was the hit and run driver who killed Myrtle who angered George who sought revenge in killing Gatsby, and we see that Daisy was also indirectly responsible.
Myrtle also has an indirect role in that if she hadn't have run into the road to try and flag Tom down (since she had seen Tom driving this same car earlier in the day and assumed he was still in it) she would not have been hit...which triggered the events we mentioned above.
Finally, we see that Gatsby himself has a part to play. If he hadn't started the affair with Daisy and then told Tom in New York that Daisy was leaving Tom to be with him, then Daisy would not have run off confused and frantic in Gatsby's car and hit Myrtle which, again, started the whole tragic chain of events.
So the direct answer to your question is: All of the people you mentioned were in some way responsible for Gatsby's death.