... I am wondering if I will use the 70-300 or will I be lugging it around the world and not using it because everything in London and Paris will be close and crammed in, negating the need for longer focal lengths.
I haven't spent a lot of time in London or Paris, but I have spent ample time in other major cities (and I'm in Boston daily). The 70-300L is part of my urban travel kit precisely because
everything is 'close and crammed in'. The longer focal length allows you to isolate subjects – people, architectural elements, etc. – from all that clutter.
FWIW, the rest of my kit comprises the 24-70mm f/2.8 II and the 17mm and 24mm TS-E lenses. I also take CPL filters, and 10-stop ND filters so I can blur people out of scenes, but that obviously requires a stable tripod. For your kit, a 77mm filter with a 67-77mm step up ring would work well.
I used to take the 16-35/2.8L II and TS-E 24, sold the 16-35/2.8 planning to get the f/4 IS version, but opted for the TS-E 17 instead. I like the TS lenses to correct verticals when shooting architecture. You can correct them in post, but you'll get cropping of the FoV if doing so, so if that's your intent be sure to frame very loosely.
I am also considering taking the 85 1.8 for some wide open portraits, but once again I am not sure whether it is worth taking it.
The 85/1.8 would allow you to get strongly OOF backgrounds...consider that if you're taking portraits while traveling, you might want that background blurred slightly but still recognizable (as opposed to explaining to everyone that the vertical blur is the Eiffel Tower and not a telephone pole).
Also I will be purchasing a Gorillapod SLR Zoom to take with me.
Good idea - you'll want to be in some of the pictures!
I will be travelling with the wife, who can't understand why it can take longer than 15 seconds to take a photo, and why would you bother walking around to find another angle. So I am normally under time pressure which also means that constantly swapping lenses just ends up annoying the wife, which in turn end up annoying me. But I am sure a lot of you can understand that situation !!!
One option to consider is taking just two lenses on any given outing (24-105 and pick wide or long depending on agenda). A Lowepro Lens Exchange 100 AW will hold the 70-300L (and thus any of your other lenses), and the design makes lens swaps easy (it expands to hold both lenses while swapping). Wear it on a sturdy belt (or on a Lowepro Light Utility Belt with two of the three 'pads' removed).
Have a great trip!