You can try Filters > Distorts > Value Propagate...
If you are on a transparent layer select "More opaque" to increase and "More transparent" to decrease
As you can see on the screenshot you have many option (More white/More black/colors and so...)
This will increase or decrease by 1 pixel (once you clicked OK, just do Ctrl+F to repeat until you are satisfied with the thickness)
If things start to look funky, un-check the different check boxes
More about this underrated tool https://docs.gimp.org/2.10/en/gimp-filte...agate.html
If you are on a transparent layer select "More opaque" to increase and "More transparent" to decrease
As you can see on the screenshot you have many option (More white/More black/colors and so...)
This will increase or decrease by 1 pixel (once you clicked OK, just do Ctrl+F to repeat until you are satisfied with the thickness)
If things start to look funky, un-check the different check boxes
More about this underrated tool https://docs.gimp.org/2.10/en/gimp-filte...agate.html