I think you've identified a key issue with one of your comments - you will be spending most of your time on the bike. This, by itself, is a clear indication of what other clothes you will require - virtually none.
I don't know what you ride in, but if it's leathers or a purpose designed riding suit, then that's what you will be in 75 % of the waking hours. So, at least as concerns clothes, all you need is a four days to a weeks supply of appropriate underclothing (type depends on the weather).
Coolmax type undergarments are great (for warm weather) because in addition to the function they serve in keeping you cool, they take up virtually no space, can be washed in a sink in about five minutes and are guaranteed to be dry by the next morning. This helps keep down on the packing.
If you aren't really going to be doing a lot of activities off of the bike then a couple of pants, two or three shirts, shorts, and a pair of comfortable shoes and your set as concerns being dressed. You wont need more - by the time you get done with a long day's ride you'll probably walk into the local diner still wearing your riding gear, have dinner and then head to the hotel to hit the sack.
As for tools and that kind of stuff - tire kits are of course a given, and a minimal set of tools for minor repairs, but lets be real here, the GT is way too complex to do any real roadside maintenance, so no point in packing as if you intend to break her down and rebuild the engine on the roadside. You're not going adventure riding in the Sahara desert, take a cell phone and the AAA card instead.
As for weather changes, again, it depends on what kind of gear you have. I use a gore-tex lined leather suit. I live in Spain where in the summer it gets to over 100ºF and in the mountains (e.g. the Alps - yes, I know they're not in Spain but that's where I go often) can end up with snow - as you can see from the pictures below. These were taken last July where we had days with temperature swings from below freezing to near 100º in a matter of a few hours. I only needed that one suit (no rain gear, no stopping to change under an over-pass while 18-wheelers come screaming past). To keep warm I use a thermal silk or merino wool shirt and an electric vest. To keep cool a Coolmax type, tight fitting t-shirt and open up the jacket to get airflow going.
Definitely need two or even three types of gloves (regular riding leather, rain gloves and I also use some short, very vented summer gloves for when I have to ride through a city at slow speeds and need ventilation). Good thing is they don't take up much space.
A replacement shield for the helmet is also a must for me. I generally take a clear and dark shield, but mostly have a backup because a bad scratch in the wrong place can be a nightmare.
One final suggestion, put all the stuff that you might need to access during the day in the same case - it avoids you from opening everything up looking for that electric vest, different glove, etc. That way the other bags only have to be opened once a day - at the hotel.
Here, these pictures below is how I have travelled - two up - for trips of 3,000 miles (to-from the Alps) over two weeks. When I do long trips solo, I just take the two sidecases (and not even expanded). And keep in mind that on the two-ups the S.O. brings along a damn hair dryer!