Most of us are not not from India and can not really give good advice about Indian universities. It would help us if you tell us more specifically what mathematical background you have ("mathematics of graduation level" could mean anything) and what you want to do with your degree (serious research? teach in a small college in India?). Do you have an idea as to what kind of math you like (applied, pure, statistics?)
Here is what little I know about mathematics in India. You should take everything I say with a pinch of salt, as I have never lived in India and have no mathematical contacts there. As far as I know, the best place in India to pursue graduate studies in mathematics is probably the Tata Institute for Fundamental Sciences (TIFR) in Bombay. The Chennai Mathematical Institute is also very good, I am sure. For statistics, and possibly applied math, the Indian Statistical Institute in Kolkata has a good reputation, I think. You should also consider the IIT's, the Harish Chandra Institute (I don't really know about their mathematics, but they have a very famous string theorist there: Ashoke Sen), and Bhaskaracharya Pratishthana in Pune. There might be others, that I have missed. TIFR is probably the best, by far. You should find out if these institutions really require GRE scores before taking the tests. I suspect that they do not.
If you want to do your Masters or PhD outside of India, you might have to take the GRE tests, you will have to find out from the institutions you are applying to. Scoring well on the subject test might help convince people that you know your stuff, despite having a non-math background. You could also do your MA/MSc in Germany (you can also do your PhD there, but your chances of getting in are extremely slim without even a math background. After doing a masters there your chances are much better, depending on how you fare). GRE scores are not required and education for foreign citizens or nonresidents, while not free as it is for residents and citizens, is very cheap compared to the US and UK.
You should also check out the Abdus Salam Institute for Theoretical Physics in Trieste. They have a program in math and have an excellent reputation.