Do you have the "Resynthesizer" plugin ? https://github.com/bootchk/resynthesizer
If so, select the watermark, then Filters > Enhance > Heal Selection... (try the default values first Ctrl+ Z to cancel if you're not happy with the values)
G'MIC does it as well > https://www.youtube.com/watch?v=VOPHbSgJUSI
If so, select the watermark, then Filters > Enhance > Heal Selection... (try the default values first Ctrl+ Z to cancel if you're not happy with the values)
G'MIC does it as well > https://www.youtube.com/watch?v=VOPHbSgJUSI