I did this tour in November 2009; I live in Colorado, & did not find the weather unusually cold for me, although China is definitely more 'humid' than Colorado. The route of this tour will start out colder, & get steadily warmer as you progress into southern China. The coldest day was on the Great Wall, outside of Beijing; I wore a fleece jacket with a t-shirt & a lightweight sweater underneath, & was warm enough (a neck scarf would have been nice too, tho). It can also be 'chilly' while on your river cruise on the Yangtze. I never needed anything warmer than that fleece jacket. A good rain coat/poncho is a 'must' that time of year; it can rain pretty hard, & if you're holding an umbrella you can't be taking photos! Good walking shoes (not brand new) are a must, as many times you're walking on un-even pavement or ancient stone pathways (hiking boots not needed). Good supportive sandals (Birkis, Keens, Merrells, Tevas, Rockport, etc) are also good in the warmer areas. A good sun hat (or small travel umbrella) is a must for the hot, sunny days. China is very 'casual' in dress - polo shirts, capris, longer shorts, 'nice' t-shirts, khakis & cotton slacks are all OK to wear. I brought a 'polo shirtdress' with me, & wore it several times; it was cooler to wear than pants, & casual enough to wear sneakers or sandals with it. The air conditioning on the buses GCT uses within the cities was pretty cold; I always wanted a cardigan/fleece jacket with me for the bus, & would leave it there when we left to walk around a site.
HINT: bring one or two of those nylon 'reusable' shopping bags from your local grocery store; they're easy to stuff into a purse or daypack, & are more comfortable to carry than the inevitable cheap plastic bags your purchases will be put into, esp. when shopping from the 'street vendors' - a fun cultural experience in itself!
NOTE: unlike many countries, you should NOT bring any small-denomination US $$ with you to China; noone will accept them on the street or in the hotels as 'tips' or for buying inexpensive items. Instead, you should use ATM's (they're EVERYWHERE, & I had no trouble using my Colorado bank checking account debit card) or exchange money at your hotel's front desk (however, be among the first in your tour group to do so when you arrive, as they usually run out of Chinese yuan). Also, the Chinese are not interested in bartering for items you may bring with you to trade - they want cash!
HINT #2: if you like 'cream' in your coffee, bring your own instant creamer. It's not always available. If you need decaff coffee, bring some of your own also. Lots of good tea in China (!), not so much good coffee. (note that some tea has more caffeine than coffee, if you really need that buzz in the morning)
Bring LOTS of memory for your camera - the photo opportunities are endless. I love taking photos of children when I travel; the Chinese parents are very proud of their children & were very gracious whenever I asked if I could take a picture of their child. Everyone enjoyed seeing the photos in my viewer afterwards.
Enjoy! Learn a little Mandarin from the Internet before you go - they were very pleased with my small efforts to say 'hello', 'please', 'thank you', 'goodbye', etc.