You may need to get a pack of assorted spacers, and use those that give you an even gap all round. If the width is 300, then you'll have 2 spacers on the bottom, and then put them on the sides about 300mm up from the bottom, then fit the top ones, and then fit them on the sides near the top.
You've given the gaps at the top as being 2mm, and 6mm, but you haven't said what the gaps at the bottom are. And if you have such a difference, it suggest the glass is leaning over, or the frame isn't straight.