Adafruit_GFX was intentionally removed from 1.53, because the older copy we had was causing a lot of conflicts with several of Adafruit's libraries. Those conflicts have been an ongoing problem, so rather than update GFX again and (probably) have these problems return as Adafruit continues development of this library, I decided to remove it from the Teensyduino installer.
The intended use is to install Adafruit_GFX using Arduino's library manager. In the beta testing for 1.53, we installed the latest Adafruit_GFX and checked it works with the remaining libs in Teensyduino which use it in some way.