We do not yet have a Gàidhlig spell-checker as such, but meantime Roy Wentworth has produced something which works very well for Microsoft Office. He has collected more than 20,000 Gàidhlig wordforms in a "custom dictionary" which can be used as a "supplement" in the English spell-check.
To set it up, save the following file
to the desktop.
Then in MS-Word, go to:
Tools > Options... > Spelling and Grammar > Custom Dictionaries... > Add...
Choose gd.dic from the desktop and click "OK", and "OK" again.
"Check spelling as you type" on;
"Always suggest corrections" on;
"Check grammar as you type" off;
"Check grammar with spelling" off.
And that's it! (You can now delete gd.dic from the desktop to tidy up.)
Remember that this is not a true Gàidhlig spell checker. Formally, it is just a supplement to Word's English spell-check. So do not “Set Language” to “Gaelic (Scotland)” or else Word will think that no spell-checking is possible. If you notice that spell-checking is not working in a document, or in part of a document, you may have to do "Edit/Select All" and "Tools/Language/Set Language/English".
Here are gd-utf8-dic and gd-macintosh.dic and gd-unicode.dic (gd.dic in “utf-8” agus “macintosh” agus “unicode” encoding instead of iso8859-1). You perhaps need one of these for your system instead of gd.dic. For example, gd-unicode.dic seems to be the one you need for MS-Office 2007 air XP.