If you are looking for a simple text editor that can help you in managing your text with GTK+ toolkit, then Geany is the product that you need.
It provides you with an Integrated Development Environment. The application is quite easy to use because of its lightweight and small download size.
Through this application, you can get the Syntax highlighted, codes folded, and visualization f symbol lists. Apart from these features, it can also manage automatic completion of the file name, construction of snippets, and compilation & execution of the code.
You can also expect simple project management from this application too. If you are looking for specific file formats to be edited through this application, and then you should know that it supports multiple file types. Some of these include C, Java, PHP, HTML, Python, Perl, Pascal and many others. Additionally, you can run Geany on each platform that has the support of GTK libraries.
One of the simple text editors with multiple features including support for various file formats is Geany. You can use it easily with the GTK library.