How to understand the style 'auto' ,'gl' and 'nongl' in

Hi All,
Recently I came up with the file and i was confused with the
following code:
p = gr.prefs()
style = p.get_string(‘wxgui’, ‘style’, ‘auto’)
if style == ‘auto’ or style == ‘gl’:

elif style == ‘nongl’:

My question is what does the style ‘auto’ ,‘gl’ and ‘nongl’ mean in the
I’m new to gnuradio, any reply would be much helpful for me.