The minimum width and height of the window depends on the minimum width/height of the widgets inside, so rearranging/removing some widgets can allow you to lower that minimum. Note that the way the HP/VP containers are used in gmb makes it a bit misleading, as even though the HP/VP container has a minimum width/height that depends on their 2 children, they allow the user to freely divide the width/height between the 2 children ignoring their minimum, I'll probably add an option to change that (not sure what the default will be).
That said, resizing a large layout such as "Queue, Library & Context" down to a width of 300 pixels is a bit extreme, and require a more radical change to ignore the minimum width of the widgets. Fortunately I added an option in v1.1.x that allows a layout to override the minimum width/heigth of any container/widget, and I found that simply using that option on the toplevel container do what you want.
So you have to use the 1.1.x version, and add the options minwidth=300,minheight=500 to the toplevel container, for 'Queue, Library & Context' it is VBmain :
VBmain = (minwidth=300,minheight=500) HBmenu 5_HPBottom
(options for containers are put in brackets before the contained widgets.)
Let me know if you need help editing the layout.