The usual temperature in Guilin in Dec is about 7~15℃. It is a little cold. Knitted sweater, jackets, plus one coat are OK. If you are not adapt to low remperature, you can prepare one down jacket in case the temperature change lower. Anyway, better to pay attention to the weather forcast before you go to Guilin and then decide your package. Guilin Weather(https://www.travelchinaguide.com/climate/guilin.htm) may give you much more useful information.