06-06-2022, 08:53 AM
Looks to me like a noisy jpeg image 
However
(1) Pick a colour (2) Make a tile (say 400 x 400) & fill (3) Apply noise (try RGB) Filters -> Noise https://i.imgur.com/w1VR80r.jpg
(4) Apply a Gaussian blur Filters -> Blur (5) Make seamless Filers _> Map -> Tile Seamless (6) Export as a Pattern file. https://i.imgur.com/shODkek.jpg
You can replace the Gimp Tile Seamless with a third part plugin 'resynthesizer' and get a slightly better blend. Not really necessary for this one.
However(1) Pick a colour (2) Make a tile (say 400 x 400) & fill (3) Apply noise (try RGB) Filters -> Noise https://i.imgur.com/w1VR80r.jpg
(4) Apply a Gaussian blur Filters -> Blur (5) Make seamless Filers _> Map -> Tile Seamless (6) Export as a Pattern file. https://i.imgur.com/shODkek.jpg
You can replace the Gimp Tile Seamless with a third part plugin 'resynthesizer' and get a slightly better blend. Not really necessary for this one.