Well, there's nothing particularly 'natural' about her being taught to read, for that matter! Lady Gwyneth is in a somewhat precarious position holding the manor anyway (as a widow) and is seen as unusual in wanting her two chamber-maids taught to read. I don't think Gatty does (or learns) as much in the way of writing, but the priest teaching her to read does teach her her letters. Arthur is supposed to be able to write as his father's squire, and does much more writing than Gatty.