gis:

Warning

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

Config file section

Valid in machine config files YES
Valid in mode config files NO

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

See lights: for details about the lights section.