The straights of Gibraltar are good for finding Task forces not becase this is a start or end destination for them (although it is for several of them) but becase the routes for several task forces go through here. Its random when and where task forces will be along their routes during any of your assigned patrols, so sitting and waiting at the straight of Gibraltar bottleneck as a sort of "chokepoint" gives you the best chance of coming across one of these task forces. It could be hours before one turns up, or it could be days or maybe even weeks, so its not always fruitful, its just the luck of your timing. This website lists the task forces as well as moored capital ships throughout the campaign:
http://edseek.com/archives/2005/05/0...urvival-guide/
It has grid reccomendations for several other choke points besides Gibraltar for coming across task forces.