Hi, I use KDevelop to write code on Linux. But I don't really develop classic applications (more like OpenGL and Vulkan stuff). Another great IDE for Linux is NetBeans, but it won't give you a DnD (Drag and Drop) GUI either. If you want to create a classic GUI application with DnD, you might want to take a look at Qt Creator IDE, though you will have to use Qt. I don't know about GTK (sry).
Personally, if I had to write a GUI application, I would probably use Web Development technologies with NW.js (for example the Mongo Management Studio was built with NW.js) or Electron.js (the Atom editor and Visual Studio Code were made with Electron.js). You then are able to easily use HTML and develop on the operating system of your choice. I recommend not using DnD for HTML.