....GIMP was not able to produce one as realistic as the online generator..
Probably some script that divides the image up into tiles, colour-averages each tile and then re-assembles the tiles + a bit of scaling. This is the correct forum, somebody bound to know of a script that does something like that
Not impossible to get 'almost' there with just Gimp basic tools.
If you look at the 'pixelated' avatar, although 75 pix wide, it could be reduced to 11 pixels width.
https://i.imgur.com/1l8XIdm.jpgTo limit colours, into indexed mode, Image -> Mode -> Indexed, with a reduced set of colours. The RGB image has a lot of colours due to antialiasing.
https://i.imgur.com/LMC0XGG.jpgScale down, Image -> Scale, to 11 pix wide using Linear interpolation.
https://i.imgur.com/S4MHEve.jpgBack into RGB mode, and scale up to required size, using interpolation None.
https://i.imgur.com/23fpRTY.jpg