Finding distance for winding road on map

[denzjos]

Maybe not an answer to the question of carljong to use gimp for measuring a track, but I always use the free software MapSource from Garmin to plan my tracks. One can download and use free maps for use with this software (ex. streetmaps, velomaps,etc...). For Mac there is basecamp from Garmin but I found a freeware (now donationware) for making tracks on the Mac. If you plan to convert many tracks in the future, the use af such a program is an easy solution. Just draw the track from the image on the map and the length of the track is calculated.  

https://macdownload.informer.com/trailrunner/