I was actually working with some really large STL files this morning and my gcode files ended up being around 800MB! Thankfully it still only took S3D around 45 seconds to create them
I've made plenty of gcode files over 1GB in size, so I would say you are no where close to the limit. Let me give you a few tips. First of all, as KW mentioned, you will need to have a decent computer with enough memory available to work with the large models. I would also suggest disabling the visualization at first to make sure that you export the gcode file successfully. You can go to Tools --> Options, then click the Visualization tab, and disable "automatically load preview after slicing". This way, after the software finishes slicing, it will first export the gcode file to your hard drive. For large files, I like to do this first before I mess with previewing it.
If you are trying to preview a 1GB gcode file, you definitely need to enable their low detail preview mode, because while the high detail mode looks nice, it will require a lot more resources for these large files. So instead, go to Tools --> Options, click the Visualization tab, and enable the "Low Detail" preview mode. That will save a lot of time and resources when you try to preview it. Once you've done that, go to File --> Preview Gcode File, and select the file you exported previously.
I use this method all the time and it works great