What is the best way (GDS, ariline..etc) for me, a soon-to-be travel
agent, to get the cheapest international fares for my customers to
India?

From ?

find a good consolidator/wholesaler....call the airlines you will be
using most & ask for who consolidates their air for them...they will
give you some companies who have contracts with them.  Over all thats
the easiest way to go...BUT....there is NO easy answer....example:
sometimes airlines throw out super duper sales that no one can touch.
sometimes consolidated fares as amazing because the published fares are high
sometimes the GDS is cheaper
sometimes buying on-line is less.

as having 10 years industry experiance....no one way is fool
proof...you just got to check around....but....more often than not
consolidator air cost is less than anywhere else, has less
restrictions, and if you develop a good relationship with an agent at
the consolidator company....she/he can become your best friend.

i don't sell travel to India, so i can't giveyou a recommendation to a
consolidator to that part of the world....but...you can ask a fellow
agent/co-worker & i'm sure they can help.