Gambas might be a very good language and environment, but it does not have this feature:
Select the Form Tools.
Select one of the object, e.g. a button, place it in the window.
Use F4 to display its properties
Double click the button the see the code, which will be executed when you click it.
Enter a code line e.g.PRINT "Hello World. It is now: ";now
In the project window find the start button, press it (or press F5 key).
An Immediate Window will be created and shows the result of the PRINT command in the debug windows.
The GUI design is clearly not being done in the running application.