I am able to compile and create executables with Vala programming language and Gtk. The GUI executables run okay from command line with command:
./myGuiAppl
However, when I double click on them in file managers, they do not run. It seems they are seen as libraries and not as executables. Where is the problem and how can it be solved?
This is probably a design choice by file managers. On Windows it is normal to run .exe files from Windows Explorer, but on Linux applications are usually run via .desktop files from the desktop environment or from the terminal.
There is nothing you can do from a programming standpoint. Ask the vendor of your file manager if you think this should be possible.