If you are looking for a simple text editor that can help you manage your text with GTK+ toolkit, then Geany is the product 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.
You can get the syntax highlighted, codes folded, and visualization of symbol lists through this application. Apart from these features, it can also manage automatic completion of the file name, construction of snippets, and compilation & execution.
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, 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.