Yes I use GIMP and I haven't found a good way to use it for white balance.
I just discovered a while ago that I could pull up both raw files to compare in DPP, and I was able to get very close.
Sometimes I just shoot jpeg snapshots (because I don't want to work all the files) and skin tones can be a problem. Most of the time outside they are ok. Once the sun gets quite low my camera tends to shoot more blue.
