1. EXCEPT FOR is not acceptable in the GMAT as an alternative to EXCEPT.
EXCEPT FOR is often used to say 'if it were not for' or 'if it had not been for'. It is therefore, in those cases, an implied conditional form.
Look at this example: Except for its large and powerful heart, a hyena would not be able to run for long distances in pursuit of prey. (The idea here is that if it were not for that heart, the hyena would have to stop after running a short distance.)
Here is another: Except for the influence and example of Mahatma Gandhi, the transition to independence in India might have been much more turbulent. (The idea is that if it had not been for Gandhi, achieving the independence of India would have involved problems that were in fact avoided.)
2. EXCEPT FOR can also be used [...]
3. EXCEPT, on the other hand, is used [...]
4. EXCEPTING is the present participle of the verb TO EXCEPT, and could also appear as a gerund. It is used [...]