You get circles to show where to dgun and it turns your com in the direction of your mouse pointer using CMD.ATTACK (or if you're moused over a unit it uses CMD.RECLAIM or CMD.REPAIR as applicable)

Download v1.2: http://pw3n.net/widgets/unit_dgunhelper.lua
If anyone wants to go ahead and improve this feel free I just wanted to make a quick widget cause the idea popped into my head but I didn't want to spend hours perfecting it :)