Offset Filter
Here is an example of it in another program.

What it does is take the open image and
rearrange it so you can create seamless
images or check to make sure they are seamless.
Following example is how it works...

Open image

Size of image is 435 pixels by 192 Pixels.
once open and applied, it does this by default.

It doesn't create a new layer but, uses the same layer.
Otherwise, you have to create 4 layers and use the
align distribute and merge down. The edge modes are
for the alpha channel. I would write a script but don't
understand how to code it right now.

Have you tried the 'small tiles' filter. Found under 'map'.

Also there is a few different things in the GMIC plugin under Arrays and tiles

Did some looking around the web and it has one already... Ooops! my bad.

The offset technique, step by step

Almost every image editor has an “Offset” or “Wrap Around” function
(GIMP has the same under Layer → Transform → Offset).
Here's the workflow:

Start with a square canvas — say 2000×2000 px — and place your motifs, keeping them
away from the edges for now. Apply Offset by half the width and half the height (1000 px
and 1000 px), with wrap-around enabled. The four corners now meet in the center,
exposing any seams as a cross-shaped join.

Paint, clone or drop new motifs over that central seam to blend it — but never touch the
outer edges, which are now guaranteed to tile. Optionally offset again to double-check,
then flatten. Your tile is seamless. The magic is in step 2: because the offset wraps pixels
around, the outer edges you can't see are the same pixels now meeting in the middle.
Fix the middle and you've fixed the edges.