The Gimp Animation Package, or GAP, is a plugin that lets you do video or animation in Gimp. Like most plugins in Gimp, it is both very powerful and very unfriendly. If you have done animation in a program like Flash, you are used to having frames laid out along a visual timeline. The timeline in Flash has a bracket to show the onionskin range, which is the frames you will see overlaid on the current frame to help create a smooth animation.
None of this is readily apparent in GAP. Each frame is a separate Gimp file, and GAP sees that they’re part of the same animation because the file names end with successive numbers. There is an onion skinning item in the GAP menu (cleverly disguised as Video), but it doesn’t seem to do anything until you configure it correctly.
To configure onion skinning in GAP, go to Video » Onionskin » Configuration…. The onionskin configuration in GAP is exquisitely flexible and confusing. Here is how to configure onion skinning in GAP for a couple of common scenarios. Note that I’m using Gimp 2.8.2.