which subway do I take from Grand Central Station?

Take the 7 train across to 8th Ave and then A/C/E downtown to 14th St. Then walk west.

Take any of the trains that will bring you to 14th St/Union Square. Then switch to the L train going to 8th Avenue & 14th St. Go out of the system & walk west. The Whitney is near the Hudson River & by 14th St.

You can take any train A/C/E or 1/2/3 to 14th street and walk west from there. Google Maps should help with the walking directions