Like aforementioned hug the African coastline, then when you're about a few degrees east of the west edge of the harbor plot a NE course to the western side of the harbor. I've had good luck with that route as it does go through a minefield but have gone that way dozens of times so can assume its a safe gap in the field. To play it really safe I travel into Gibraltor itself at 1 knot submerged at 14-15 meters.
Once in the harbor head towards the NE portion, you'll usually find the Nelson there and can deep6 her with 3 eels. Though the harbor is crawling with DD's, if you hide behind the wreckage you can usually surface long enough to refill your O2 if needed and get a little recharge on the batteries, just don't stay too long, eventually someone will find you.
Be sure to watch out for subnets, they're all over the place! Gibraltor is probably my personal favorite harbor to raid, its dangerous, can involve some serious patience dealing with dc'ing, and has nice goodies once you're in
