Difficult to say if it was the right decision, as there was a case to be made for all three of them.
But, as Lord Sugar put it, he knew about Bilyana (her errors and gob) and he didn't know about Katie yet (though that didn't stop him in the past).
Gabrielle probably deserved to stay for her boardroom defence, though she should have handed over the finances to Jane or Bilyana.
Also, the editing (and Nick) makes it look like Katie was invisible during the task, but did you guys know there is also a Scottish lass competing (the bridal shop owner)??
As for looks, which seems the main reason people are upset about this firing (let's face it, if you're fired can almost fill an entire episode of your failings then you didn't do well), I had all three ladies tagged before the show started so the end result would always be the same.
Strongest candidates seem to be Jade and Duane at the moment.