Missed approach climb gradients should be predicated on engine out performance for Perf A aircraft. (In the approach configuration) I think there is a JAA requirement for this to be greater than 2%.
However, as the aircraft will normally be at or below max landing weight when going around, which will typically be 10% or 20% below MTOW, the missed approach climb gradient should be somewhat better. Our ops manual for 757/767 give a minimum OEI climb gradient for missed approach of 5% at MLW.
If in doubt, use the higher minima!