Hi,
I am currently in the process of looking for schools and applying for PhD programs as well. (Management)
From what I have seen: absolutely no chance to get into a program with a GMAT score below 600.
There are a few decent schools out there who accept scores in the low 600s but I think the rest of your application has to be brilliant in order to be considered with such a low score.
Additionally, if you have a GMAT score of at least 650 you usually get better funding.
I have taken the GMAT twice this year for my PhD application and I will take it one more time because I am not completely happy with my 630.
The schools I am looking at (and these are NOT top 10 schools but decent schools in my opinion) have an average GMAT score of 650.
My recommendation is: definitely take the GMAT one more time. It is going to be hard to get into a fully funded program with a 600, especially since you are Indian. I think the competition among Chinese and Indians is EXTREMELY high for these programs.
What schools are you looking at?
P.S. I am guessing that since you are from India you are probably stronger in quant than in verbal. Hence, stick with the GMAT and don't take the GRE. I was once told that the GMAT is easier for non-native English speakers. Little did I know that they were referring to a certain group of non-native English speakers, namely the ones who are awesome in math. I don't belong to this group unfortunately.
P.S2: I agree with SoutCity: it would really be best if you could get at least a 650. This seems to be some kind of bottom for most schools. Sadly it is true that schools pay the most attention to the GMAT score. I have a mentor who was once on the admission board at her university and she confirmed that the first thing they looked at was the GMAT and then GPA. Letters of recommendation don't matter as much, most of the time they don't even read them if your GMAT score doesn't meet their expectations.