> in Spanish, gender is pretty easily determined from the word ending 99.9% of the time unless it's a Greek neuter, and there are only a few of those.

Isn't that 99.9% referring to the reliability of spelling heuristics for the words that match a heuristic? For example, the -o and -a spelling heuristic reliably indicates gender something like 99+% of the time, but only about two thirds of Spanish nouns end in -o or -a. There are a few other suffixes with string gender indication, but even then I think you can only account for like 75% of Spanish nouns.