I was thinking to make a level editor. This will definitely need some sort of GUI, but working on the iso engine poses already enough challenges to me and I think that making a GUI framework from ground-up is not worth it, if there are better options available.
After wasting most of the last Saturday trying to make SDL render into a GTK widget, I realized that that was not the way I should go. Even though I've managed to get something working in the end, I now think it is better to have the GUI inside SDL, not outside it, as a GTK solution would do it.
Some queries with apt-cache returned guichan and paragui as the possible candidates for what I had in mind. Of the two, guichan looks to be only one that is still actively developed, so I will probably end up using it.