How do Heartless Hidetsugu and Gisela interact?
You didn't miss anything. Wizards preempted this with their ruling on Gisela


  Gisela doubles damage dealt to opponents and permanents your opponents control from any source, including sources controlled by those opponents.


So it's pretty straightforward: double of a half is a whole. As you noted, the rounding could put it at 1 instead of 0, but you're playing red: surely you have 1 point of burn somewhere in your deck, right? :-)

There's no rules that say "Super-powerful combos aren't permitted".

So yes, your opponent would be incredibly unwise to tap it, although you could always Act of Treason it and tap it yourself!