To display the gobo in 3DView, there must be a PNG file (128*128 pixels), with the same name in this folder: "YourSoftware/3DView/gobo/"
Here is an example:

Note:
"ok_for_3DView" is an example of gobo folder name (you can create/use another folder).
"YourSoftware" is an example of name of our software.