Most important thing is to make a backup of your table before you use an update query if you're testing things :). Copy/paste the table and get data and structure, then you're OK if you accidentally do something.

First make your query that only returns your values you want. Bring in the fields and put criteria for Category to be office or fuel, then on the cleared field update to false and on the addgst field update to true

Yep I agree 100% with Adam.
Get in the routine and make it habit.   Every morning before I even open the db - I Back up the whole project.
Then through out the day as I'm making changes I make quick backups of just a single form or table or query, etc.
Especially my code.  You never know when you want to scrap what your doing and go back to the way it was.

I also echo Adam's suggestion on the query.  Make a select query first to test your IIF.  Then change it to an Update Query.

IIF  function -
Give it a shot.  If we write it and just give it to you what did you learn for next time?  You got this!  Look how far you've come.
If ya get stuck post what you have.  We'll help.

Glossary IIF Function

IIF Function

Thank you Adam and Scott.  Ok I will give it a try.  :)

Wooo Hooo!!! :)  

I did it!  But I did it a bit different than I had originally planned.
I added a field to my CategoryT (IsTaxable) instead, so I can use it for new transactions too.
Added a Check Marked to the Business Fields (So I didn't have to add 20 "OR" categories to the Criteria)
CalcGST (Update to True) - Criteria (IIF([IsCleared]=True, IIF([IsTaxable]=True,False))
IsTaxable Criteria to True

I can't believe it worked.  Thanks for the push Adam and Scott.

Sweet!  Sometimes the small victories are the best.  
Glad you pushed through and figured it out.