I do not think this is a stupid question at all! This is one of the most difficult aspects of literature to teach, discuss, or write about. Most people confuse "voice" and "tone," and it is hard to figure out the difference. "Voice" tells us who is speaking, so, for example, even without "Smith said," or "Jones said," we can often figure out who is speaking (or thinking.) Tone conveys the attitude of the person who is speaking, and this is chacterized by syntax, word choice, and content.
Now, in The Great Gatsby, we have Nick as the primary person whose tone there is to discuss because we can see into his thoughts, while with the other characters, we have only dialogue and action to decipher them. Nick wants to be seen as a somewhat cynical and "with it" kind of person, but part of him longs for a world of true love and heroism. His tone reflects these contradictory aspects of his character. When we see him assess Jordan Baker, for example, or Tom Buchanan, we see his cynical take on the world. Even when he assesses and discusses Daisy, we know that he sees her as she is, a very material girl. But when he starts to see more of Gatsby and watches Gatsby's love for Daisy, as he reveals his thoughts to us, we can see that he is longing to have a hero to admire, and his longing for true chivalry and true love. If you read the novel carefully, paying particular attention to the opinions and thoughts Nick shares with the reader, as opposed to what he has to say in dialogue, you will see the difference in tone.
I want to tell you once more that this is not a stupid question by any means. Good luck to you in your quest for tone!
Fitzgerald tends to write in a very descriptive, some would even say flamboyant, style. Because 'Gatsby' is in the first person, maintaining the sort of complexity and formality Fitzgerald specializes in would be difficult without making the narrator seem pretentious, or affected. But Nick is portrayed as a romantic, a dreamer, someone who wishes for something greater than what he has. His opinion of Gatsby and Daisy's love affair is related to his own views of love and romance; much like Fitzgerald himself, Nick has some very definite ideas about what love is, and perhaps more importantly, what it can be in its most noble form. He also sees something noble and admirable in Gatsby, despite his flaws. Nick is also a bit of an outsider, and feels privileged to be able to attend parties with rich socialites like Daisy and Gatsby's friends. Because Nick is prone to romanticizing the people and events in his life, the tone of the novel becomes very grand and sensual, and this allows for a great deal of stylistic and tonal contrast when the events and persons described aren't terribly noble or admirable.