The easiest way is to install Gambas from your Linux distribution.
If Gambas is not yet contained in your distribution then download the distribution
from http://gambas.sourceforge.net
First Steps
When finished the installation then use Gambas:
In a terminal window type "gambas2"
Gambas will open, select new project.
Select "Create a graphical application Qt", click next,
Find a directory where to put the first project, click next,.
Enter a name for your first project, click OK.
Close the tips of the day window.
Right click Forms, New, Forms...
Find the Toolbox window on the workspace. (type F6 it is not visible)