When you are posting a message, in the bottom right of your post you will see a button that says Go Advanced. Click this, it will show more options and allow you to attach a your log file.
In Advanced view you will see the below toolbar. Click the paper click icon (highlighted).
You will now have another window on screen, in the upper right corner click the Add Files button, another window will appear, click the Select Files button.
Browse to your GB folder and click select the debug file, click the Open button. Now click the Upload Files button.
Click the Done button and thats it, your log file is now attached to your post. Submit your post.