I often type fast and make typos where letters appear in the wrong order. I would like to have auto correct fix words like “hte” to “the” and so on. Also I want to add other words to dictionary. Is this all possible with Gmail on the desktop?
There are a few ways as already stated. Another way is to use something like Thunderbird to Send and Receive your Gmail, then just use the spell checker.
Chrome (which I believe you are using) has an automatic spellchecker that usually underlines misspelled words . That is more than handy, and you can right-click and add to the dictionary.
Here's the support page - http://support.google.com/chrome/bin/answer.py?hl=en&answer=95604
Use exAutoCorrect To Make Spell Checking in Gmail Faster & Easier [Chrome]