This question relates to practice exam 1 from the book asking for the cloudbase for BWL from the GAF 6.
The GAF in the remarks section gives us:
BWL: [ELEV 2200FT] SHRA BKN ST 2200FT [CLD ON GND]

Since this relates to a particular aerodrome the height could be interpreted as AGL, just like we would for a METAR.
However since "CLD ON GND" has been added, it is pretty clear that clouds are on the ground.
This leads me to believe that in this case the height of the cloud is given in AMSL, which would also make sense since this is a GAF.

I tried finding out more from the BOM guides, but so far haven't found a definitive answer on if any heigh given in a GAF is always in reference to AMSL or if there are cases where they could be provided as AGL when a particular aerodrome is referenced.