Mutt,
A 2.5% missed approach climb gradient is the standard for determining minima for an instrument approach. Most aircraft can do considerably better than this and those that can't will hopefully be all too aware of their aircrafts deficiency and higher minima will be imposed.
Some airfields have different minima for different missed approach climb gradients, Dalaman has ILS minima of 1210 feet for 2.5% gradient and 210 feet for 8%! Regretably with no intermediate values to allow lower minima for say 5% gradient.
The 5% gradient published for 757 and 767 at MLW is one size fits all. The aircraft will manage at least this figure at all temperatures with a crook donk.