Ok, I'm no English major but I'll take a shot
at supporting your thesis. I think one theme that runs through the book is how Nick often doesn't state what he knows, doesn't tell people's secrets, which then leaves others with false impressions, which can have serious consequences. In a nutshell: by not saying anything when he could have, he's lieing.
Two simple examples: when he doesn't reveal that he knows about Tom's affair, when he doesn't reveal that he knows Daisy was driving the car, or even at the scene of the crime when he doesn't tell the police everything he knows (think about it, he could have saved Gatsby's life by telling the police so they got to Gatsby before the crazy husband did).
A good quote to support your thesis comes at the end of the book, when he breaks up with Daisy and she says he is dishonest, and that makes Nick angry. In the part where she says she "met another bad driver" what's she's saying is "you are as dishonest as I am, and we are a wreck together". So that quote would also support your theory.
Great Gatsby- Nick's honesty
Scott Fitzgerald portrayed Nick in an honest way, which does not mean that Nick himself is an honest person, he just addresses his reader in a way that the average person you met on the street might answer you.
He is honest to the point, that he is our story teller, and for him to be lying the whole time would make it too difficult to follow.
He doesn't lie exactly, he just tells the reader what he wants to. Meaning he may miss something out, or express an opinion that was not true at the time the event took place (Remembering that 'The Great Gatsby' is written mostly in the past tense).