Basically just the title said. The situation is basically I use a Domain-Specific Language called G’MIC, and to this day, I haven’t found a satisfactory answer to the issue of lack of syntax highlighting. At the moment, I am using KDE Kate as it’s pretty good at structuring the code with their find/replace feature, tab indicators, and multi-window support.
I would use VIM, personally. It may take some getting used to, but once you get it, it can be very useful. (VIM does have syntax highlighting, by the way)