Not complex nor dumb just not there. This an outline (because I am shutting down for the day ) Might / might not help.
Gimp does not have other than a very rudimentary crop mark (in linux Gimp) so my advice is get a copy of Inkscape. http://www.inkscape.org
Starting with Gimp, export your design as a png https://i.imgur.com/8geUOZO.jpg
Open that in Inkscape and in the Extensions menu, there is a Render > Layout > Printing marks https://i.imgur.com/loWsLq4.jpg
You then need to fit the Page to include the registration marks https://i.imgur.com/PfGdiS1.jpg
Save or Export that as a pdf or png (up-to-you) https://i.imgur.com/mSvDZxq.jpg
Back to Gimp with the reg marks https://i.imgur.com/BWB9X6w.jpg
Dont know your design, it might be a single colour but as CMY separated colours -> Components -> Decompose the registration chevrons (which are cmyk) show up. This for magenta https://i.imgur.com/DbfymKx.jpg
Always helps if you give the OS you use and the version of Gimp.
Gimp does not have other than a very rudimentary crop mark (in linux Gimp) so my advice is get a copy of Inkscape. http://www.inkscape.org
Starting with Gimp, export your design as a png https://i.imgur.com/8geUOZO.jpg
Open that in Inkscape and in the Extensions menu, there is a Render > Layout > Printing marks https://i.imgur.com/loWsLq4.jpg
You then need to fit the Page to include the registration marks https://i.imgur.com/PfGdiS1.jpg
Save or Export that as a pdf or png (up-to-you) https://i.imgur.com/mSvDZxq.jpg
Back to Gimp with the reg marks https://i.imgur.com/BWB9X6w.jpg
Dont know your design, it might be a single colour but as CMY separated colours -> Components -> Decompose the registration chevrons (which are cmyk) show up. This for magenta https://i.imgur.com/DbfymKx.jpg
Always helps if you give the OS you use and the version of Gimp.