I use gel pens on my labels and have never had a problem with fading. I do heat set them with my iron though. Just be aware that some of the colours hold up better than others so definately do a test. I bought about 5 different colours and wrote on a test label with each of them. I then heat set and followed up with a VERY rough wash i.e. I took that little label and scrubbed the life out of it
Although one or two of the colours did fade slightly, they didn't wash out as such. The other colours stayed bright and clear, especially the violet/purple one. Unfortunately I don't have the brand name with me (shhh, I'm at work).