Many tutorial sites are available for GIMP, but their quality varies. Moreover, many are too old to be useful, since they deal with versions of GIMP older than 2.4. Finding a relevant tutorial is mostly a matter of luck. Here are some interesting and generally useful sites.
www.gimp.org/tutorials/ offers an interesting set of tutorials, although most of them are several years old. No recent additions have been made.
www.ghuj.com is rather slow and complicated to use, but it contains many tutorials.
www.pixel2life.com/tutorials/gimp/ is also slow and complex, but it’s supposed to be the largest collection of GIMP tutorials on the Web.
www.gimpusers.com/tutorials/ offers many tutorials, classified in categories.
gimps.de is in English as ...