While there is a decrement in climb capability with the gear hanging, it is not so great that disaster is imminent, except in the case of terrain in the departure path. The gear down decrement for the C-5 (the only plane I have seen drag indices published) was 1.3%, I wouldn't be surprised it the gear down penalty for most planes is in the range of 1% to 1.5%. You can see, if climb gradient is limited by terrain, the gear down case might have one "scratching gravel", as my glider friends say.
GF