F&F is the official code name given to the operation by the ATF/DOJ. Gunwalker is the term used by agents in the field. It is the same operation by either name.
IIRC, the terms are NOT interchangeable. Project Gunwalker preceded Fast and Furious and was started by the Bush administration. Under Gunwalker, the straw buyers were followed and the weapons then seized when they were resold.
Under F&F there was no attempt to seize the weapons, the straw buyers were simply tracked, then the weapons allowed to continue on their way. Someone please correct me if this is not accurate.
Gunwalker was used by the Bush administration, where they actually did their jobs and arrested the straw purchasers.