The IIT Bombay campus map is attached here. You can reach the Girish Gaitonde Building from the IIT main gate by taking the fourth right.
Plane
Terminals 1 and 2 of Mumbai airport are within 12 km and 9 km, respectively, from the IIT Bombay campus. You can take a taxi using Ola or Uber from the terminals to reach the campus.
Train
LTT is the closest intercity station to the campus which is roughly 13 km. You can either take a taxi or if you want to experience Mumbai local trains, you can board from Tilaknagar railway station (which is 800m from LTT) and switch to the central line at Kurla (which is the next station) and then get down at Kanjurmarg station. You can then take an auto to reach the campus.
Tip: Install the mIndicator app if you want to explore Mumbai's local train routes.