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.