2.6 was released in 2008, 11 years ago, and since then the gegl backend, single window mode, layer groups, color management, and lots of additional features were introduced. Check out https://en.wikipedia.org/wiki/GIMP_version_history#GIMP_2.6 on what you are missing out, especially by ignoring 2.10. GIMP 2.99.2 (first prerelease of the GTK3 version) is supposed to come out this year: https://www.gimp.org/news/2019/01/02/gimp-and-gegl-in-2018/