I have not had an open back GS1300, but, from pics and my discussions with Bart, it looks to be similar to the 302 (more similar than to a 303).
Here is a pic of a GS1300 with my Nokona AMG1275. The GS isn't any longer, but is a bit deeper. The Nokona was about the same size as a 302 until I loosed up the laces. I debated whether to get a closed back 1300, the open back or the 1250USA, but finally went with the closed back version. It is currently being made. Bart told me that of them all, the open back 1300 is the narrowest and the closed back is the widest and I seem to be needing ever more room for error. That being said, the open back 1300 measures a bit wider than my 302 and with a custom web, you could get it to play much wider.
And a 302 and 504 with ball. The 302 and the GS are pretty similar.