I bought and downloaded this game it is on my self , how do I launch it?

GOG games don't run from a client. They are all independent programs, that have to be manually installed (for the moment. that might change when Galaxy, the new client, goes live)

You downloaded the installer, right? Meaning you clicked on the "download" key on your shelf? The files then should be either
- in a "gog downloads" folder, in your documents, if you used the gog downloader. When the downloader window is running, there is a small magnifying glass on the right side of every game you just dowloaded, that will take you to this folder.
or
- in the folder where you usually put your downloaded stuff (for example your "my dowloads" folder), if you used the direct download link.

In either case, you should have one or several files named "blackguards" in that folder (Most install files on GOG are 2GB or less, so if Blackguard is more than 2GB, you should have several files). One of them is an executable file. Simply run it (by double-clicking on it, for example), and it will install the game.

Once it is installed, you can launch the game the same way you usually launch any program : "my programs-->GOG-->Blackguards" in the start menu, or the shortcut that will be installed on your desktop

Hope this helps :)