My understanding is that Fu Hok Chaak is merely a much faster version of Gung Ji Chaak. There are a number of pauses in Gung Ji Chaak between sparring sequences. Based upon my training to date, Fu Hok Chaak trains the cardiovascular system much more than Gung Ji Chaak due to the lack of pauses in between sequences. However, Gung Ji Chaak trains the stance more. But in my view, both of the sparring sets are really useful if it is done at high speed so as to challenge the reaction and timing of your training partner.
I have also trained in Wing Chun intensively for the past twenty years. It is interesting to note that Wing Chun Chi Sao/Guo Sau training also starts from set sequences. They are of course not as extensive as the Hung Gar sequences. There are the Luk Sao (rolling hands movements), a set with five combination striking movements (Lap Sao) and other sequences with 2-3 movements (Pak Sao and Gum Sao), when these have been drilled to perfection then these are broken up and used randomly in accordance with how your opponent moves and any openings.
In my humble opinion, I think the Hung Gar Chaaks should also be trained in a similar manner. Once they have been trained over and over again may times, many techniques become instinctive just as in the case of Guo Sau in Wing Chun. At that time, they should be broken up into drills and sparring.
Vincent