This attribute corresponds to the GFLG_GADGHIGHBITS portion of the
gadget's Gadget.Flags field.  This attribute can be one of four values
(from <intuition/intuition.h>):

    GFLG_GADGHCOMP, GFLG_GADGHBOX, GFLG_GADGHIMAGE, GFLG_GADGHNONE

See the "Intuition Gadgets" chapter of the Amiga ROM Kernel Reference
Manual: Libraries for more information.