If you have a GL10 or some other model with a sunroof, I found that the sliding cover is painted black on top that absorbs a lot of heat from the sun and then radiates it into the cabin.  And the bottom of the cover is the hottest part of the ceiling on a sunny day.  So the solution to that is to apply aluminum faced tape to the top side and you will be amazed in the results.  Then the bottom of the cover is the coolest part of the cabin ceiling and on hot days you don't get that heat buildup radiating down on top of your head.  Which is particularly annoying when it is over 100 degrees and your A/C does not work.

You could also just get good tint on the glass, it will reject almost all of the thermal energy.

Actually not.  They come with a strong tint and that tint is useless for the prevention of the slideable cover from heating up and radiating that heat down on top of your head.