gtk:aspect-frame-set (frame xalign yalign ratio obey-child)

frame -- a gtk:aspect-frame widget
xalign -- a float with the horizontal alignment of the child within the allocation of the aspect frame container, this ranges from 0.0 (left aligned) to 1.0 (right aligned)
yalign -- a float with the vertical alignment of the child within the allocation of the aspect frame container, this ranges from 0.0 (left aligned) to 1.0 (right aligned)
ratio -- a float with the desired aspect ratio
obey-child -- if true, ratio is ignored, and the aspect ratio is taken from the requistion of the child

Set parameters for an existing aspect frame.