Hi there,
As for Marketing, I think the top programs (I'm not listing them in order because once you get into a list of the top programs, other factors come into play that help you prioritize which option[s] are right for you) are: Kellogg (obviously), Standford, Duke, Columbia, Ross, Berkeley, and Stern. There are some other highly ranked programs I have not included in the list above but do have great marketing programs (e.g. Wharton, Harvard, Booth). If you're focused on Marketing, however, then those listed are really the best options from a faculty, recruitment, etc. standpoint.
In regards to General Management, yes, a solid foundation if you haven't got one is critical. It's what will truly enable your switch out of IT and I would focus on programs that would facilitate that. For example, Booth only has 1 required course and while they provide some structure around what you're required to take, it probably isn't the best option. Tuck is just one of the schools with a fixed core (some are a semester, some a full year). If your goal is holistic general management, then you would want to cast your net wider than just Tuck and look to a variety of other criteria that are important to you.
I understand about your GPA and believe me, admissions committees are aware of this and do understand. If they don't know about your school specifically, then you can explain it to them. I, for one, came from an undergraduate institution notorious for lower GPAs. You are correct, a great GMAT score will definitely offset this. Also, the story of your work experience will help communicate your quantitative and qualitative skills. So your GPA will never be a differentiator, but it can definitely be positioned and supported such that it doesn't actively detract from your application.
GMAT: it's a good thing you're thinking so far ahead about this. You're right - crush it. Aim for 700+ and look to have a balanced score. If you can't, use the Qual section to carry your score but understand a lower Quant score will have impacts and will also need to be explained. Also, I'm not sure of your ethnicity but if you're an Asian male, that number goes to 720+ preferably. But your first priority should be north of 700, however you can get there.
The other thing is, given your timeline, if you're 15 months from applying, take your extracurriculars from decent to great! Make them unique, make them impactful. Find meaningful involvement opportunities that will support your story and help you stand out. It will also give you extra material for your essays. This can be a true differentiator.
You are definitely not doomed by your GPA - there are many ways around it but that process to determine your story starts now.
If you'd like, we can always discuss it (you can either send me a private message or sign up for a free consultation below). Knowing your story in advance will help you make a tactical plan and provide the guidelines to operate within.
. Stanford is a great program and definitely a fantastic option. 
