Weirdly, gliders are not defined as aircraft. only FB and DB are aircraft. So, no, heavy AA cannot fire at gliders in the air. Gliders in the game represent gliders that are in the process of landing, anyway. So they would be too near for heavy AA to hit at that time. There are no provisions in the rules for gliders to be hit by heavy AA 'on approach.'
[edit -- upon reflection, I think any glider losses due to heavy AA would have already been 'removed from the scenario' by the designer, as these would never have made it to the LZ to try to land in the first place]