The GTK+3 front-end is built on top of the ntdisp library, so everything it is capable of can be accessed programmatically by using the NtdIsp class. Furthermore, to avoid cluttering the interface with less used actions, some operation is not possible with this front-end. This includes, for example, clearing the flash memory without download, running a program at a specific address or downloading a portion of file, all operations possible either by accessing the library or using the cli interface.
There can be some difference because of the platform. For example, the device on POSIX systems is a file, so the input widget is a standard file chooser. Anyway, on Windows platforms this is not the case.