This plug-in will help you extract a seamless tile from an image that isn't seamless (but contains the seamless tile).
I don't know if there is such plug-in out there that behaves like this or not.
But let's say you got an image like this (which isn't seamless but you know it contains a seamless pattern).

You can get it to this by using the plug-in (vertical cut) (after making a vertical selection of where you think the seamless should be within)

Then you can get it to this using plug-in (horizontal cut) (after making a horizontal selection of where you think the seamless should be within) \on result above to get it to be a completely seamless tile.

More details about plug-in --> https://gimplearn.net/viewtopic.php?p=24952#p24952

Really cool, Tran (just watched the video).

Hi Tin.

A very useful plugin.
I will apply it to the miniaturization of all my patterns.
Thank you very much.

Awesome Tran. Big thumbs up!

Thanks, glad you guys like it.