When was GSLV used for the first time?

Geosynchronous satellite launch vehicle (GSLV) was used for the first time by Indian Space Research Organisation (ISRO) in April 2001. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Indian Space Research Organisation [ISRO Notes for UPSC]
2. Mars Orbiter Mission (MOM) – A Complete Overview of Mangalyaan Mission