This is an awesome plugin developed by Pete Nu which adds a stroke color around each layer. It's a great way to stroke text, but it can be used on other layers as well. Doing this by hand is a tedious process.
You can find the original plugin over here: http://pete.nu/software/gimp-outliner/
Gimp 2-10 broke the plugin. Selecting a color now crashes the plugin now so your only choice is to use black. I've updated the script to default to the foreground color so you need to set the color before using the plugin. Hopefully Gimp will fix this problem as it is affecting other plugins as well.
To Install:
Windows: Drop in /%USER%/AppData/Roaming/GIMP/2.10/plug-ins
Mac: Drop in %USER%/Library/Application Support/GIMP/2.10/plug-ins
Linux: ?