gis:

Warning

As of MPF 0.50, matrix_lights, flashers and leds have been combined into a single lights configuration. See lights: for details.

You would configure GIs as normal lights with subtype: gi (see your platform documentation for details about subtype).

Here’s an example from Judge Dredd:

lights:
  gi01:    # lower backglass
    number: G01
    subtype: gi
  gi02:    # mid backglass and rear playfield
    number: G02
    subtype: gi
  gi03:    # upper left backglass and slings, variable
    number: G03
    subtype: gi
  gi04:    # upper right backglass and Deadworld globe, variable
    number: G04
    subtype: gi
  gi05:    # coin slot lights & side cabinet fire buttons
    number: G05
    subtype: gi
This example is tested to be valid MPF config. However, it is not integration tested.
lights:
  gi01:    # lower backglass
    number: G01
    subtype: gi
  gi02:    # mid backglass and rear playfield
    number: G02
    subtype: gi
  gi03:    # upper left backglass and slings, variable
    number: G03
    subtype: gi
  gi04:    # upper right backglass and Deadworld globe, variable
    number: G04
    subtype: gi
  gi05:    # coin slot lights & side cabinet fire buttons
    number: G05
    subtype: gi

See lights: for details about the lights section.