Hi, As said about a month ago Gthumb version 3:3.6.1-1~bpo9+1 was accepted in backports[0] yesterday.
The NEWS file declare: version 3.5.2 * Fixed big memory leak and other minor memory leaks. If it is possible to use the backports, there will be less problems with memory leak. Enjoy! [0] - Backports instructions: https://backports.debian.org/Instructions/ Regards, Herbert