ARM produce designs for a GPU called 'Mali'. This is incorporated in many ?SoCs and thus devices. It is used in a number of devices that can run Debian.

There are abandoned free drivers (lima) available for Mali, but the binary drivers put out by ARM have been redistributable since 2016 and this can be packaged for non-free. A GPLed kernel shim driver is also released by ARM, which is eligible for Debian main (or contrib).

Mali hardware has been produced in several generations, so far Mali 200, 400, 600 and 700, with lots of minor variants. The wikipedia page is a good reference.

In practice relevant hardware is

The kernel drivers and userspace binary drivers are being packaged now (March 2017). That work is co-ordinated on the MaliMidgard page.

Upstream drivers are available from The ARM developer site

kernel drivers

There is a different kernel driver for each major hardware revision, and thus a different debian dkms package:

In fact the midgard and bifrost drivers are currently (mid 2017) identical (but have different version numbers!). This may change in the future.

userspace drivers

Each binary driver is built for a specific combination of OS (linux/Android), hardware platform, GPU generation(4x0/6x0/7x0), and graphics technology (fbdev/X/wayland/surfaceflinger). Platforms are sometimes dropped, so you might need to choose an older version to get a driver that will work on older hardware. A few plaftforms have DTB support, so one driver build can work on more than one bit of actual hardware.