GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: Basic Level

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I'm trying to make seamless tile sets in Gimp.

The theory goes, I have a bunch of small parts/images of lets say stones, and I want to copy/paste these into a larger image and layer them until I have a stone pathway tile. So if I drag a stone to the edge of the larger image how would I make the other half appear on the opposite side (so the tile becomes seamless).

I would recommend upgrading from 2.8.14 to 2.10.22

2.10 has a feature called Symmetry Painting
(found in "Windows > Dockable Dialogs)
Symmetry Painting has option called Tiling. It does what you want. Very quick and easy.

Try Filters > Map > Recursive Transform. Set the number of iterations you want and drag the edge handles inwards.