Hi Rohit --
Not quite sure what you mean by "funding" in this question, but in terms of the MBA deadlines, you'll want to go for a Round 1 application, and most of those first deadlines are in October. You don't want to be doing GMAT prep and essays at the same time, and most people require at least a month (actually more like 2+!!) to move through the essays - which means that's what you'll be doing all of September.

So definitely get the GMAT out of the way by August if you can.
Now, this assumes that you'll take the test once and nail it. The wise strategy is to actually take it even earlier, so that you can regroup and try again in case the score isn't what you hoped for. And, you need 30 days between tests... which means we recommend a June-ish first GMAT attempt if possible.
Hope this helps!!
EssaySnark