make lines thicker on already drawn object

[tmanni]

If the layer you are working on only contains grayscale values (so no colors and no transparent pixels), you can use the trick of applying the hidden filters used by Select > Grow or Select > Shrink.

Example: for growing black lines on white background, you can do:
Filters > Generic > GEGL Graph... and type:

Code:

gimp:shrink radius-x=5 radius-y=5 edge-lock=yes

or using the grow filter (for the same result):
 

Code:

invert
gimp:grow radius-x=5 radius-y=5
invert

Adjust radius-x and radius-y for your needs.