Dax I rephrase a little, it's not a fullscreen but a maximized window. Deleting workspace.bin doesn't really help. I found that this is not only a Gaea issue but also a Blender and Quixel Mixer too. Here some screenshots to clarify what I meant.
Here I set my custom window size and position
This is after I saved workspace and reopen Gaea, the window is maximized again