Spell checking and typeahead are two of my top gripes with modern software. URL or bookmark completion is ok, that is when I’m in the ‘trying to remember’ mode. But when I’m in the flow of writing, having the computer guess what I’m trying to say is incredibly distracting and annoying.
Originally, I thought gmail was running auto-spellcheck for me, but it was the browser, in this case Google Chrome.
In Chrome, you turn off spellcheck under chrome://settings/language ; uncheck the ‘Enable spell checking’ box underneath the list of languages.
(You can also get to this screen advanced settings screen by clicking on the wrench in the upper-right, select ‘Preferences’ and ‘Under the Hood’, then click ‘Languages and Spell-checker Settings’ )