make lines thicker on already drawn object

[Ofnuts]

If you used the path tool, it's only a matter of re-stroking with a wider line.

Otherwise, starting with this;

   

Apply a Gaussian blur:

   

Start the Curves tool, and make a diagonal across the white part on the right (which is the histogram of your blurred image)

   

Result:

   

The trick being to find the position/slope of the diagonal which is the best compromise between smoothness and blurriness.
You can use the Threshold tool instead (but you will have jagged edges)(Threshold is just Curves with a vertical drop). You can also use Levels.