I'll have a go at this:
An airplane is descending with a constant IAS and zero thrust. What happens to the descent gradient when the OAT increases?
No change in descent gradient.
Descent gradient (much like the climb gradient) is a function of excess drag vs weight (ie. (D-T)/W ). Since thrust is zero for our question, we can use D/W. Neither changes so the air gradient will remain the same.
Hotter (or colder) air does not imply less drag if we are flying at constant IAS. Assuming no compressiblity, const IAS implies constant EAS and constant q. Weight is constant, Lift is constant, density will increase(or decrease), but speed changes to adjust for it (TAS will therefore change), Cl remains constant (as in the angle of attack is constant).If Cl (and angle of attack remains the same) so wil Cd and Drag.