Just to be clear, even if you use your GUF on your booking as noted the LGW-JFK route does not operate with an F cabin. If operated with a four class 777 the F cabin is not normally used unless CW is oversold. Even under those circumstances you should not have any expectation to be seated in the F cabin.

bear in mind when sat in F on a four class plane the seat may be an F seat but every other aspect of the service will be CW.

Use your GUF only if you think upgrading the other flights on the booking is worth it.