I'm using gedit and the speller actually highlight the misspelled words. But when I right-click the word and choose the correct word it won't change. Also I can't choose to add a new word.
It sounds like the permissions on your options file has been incorrectly set. Can you navigate to the file /home/<USERNAME>*/.config/gedit (Go to the hidden folder
.config and right click on the gedit folder, then select properties from the drop down menu) and make sure that the permissions are set to the following: ****(swap your username for <USERNAME>)***
If they do not look like this, let me know and we'll sort it out.