You don't even want 2Gb on disk...
Use sparse arrays and store some meta data with your nodes to decrease the size (this might sound counter intuitive, but if you can already describe eg. rectangles in your format, you can store larger areas with repeating content (like water) really efficient).

Oh and do yourself the favor and don't batch everything in one file, thats going to be a pain in the ass. Create a file for every single area in your game and batch eg. 1024x1024 nodes into it