In the mid-2000s those glass effects were all the rage for website buttons and such, probably because of OS X Aqua. Here is a tutorial for the Gimp from that era (will work pretty much the same in modern versions or other graphics editors with layers): gimpusers.com/tutorials/create…

The gist is that you copy your main motive or shape into two additional layers, use one to add the gloss-effect on top and one below to add a shadow in the background. Play with the transparency of your layers till you've got the desired effect.