While strictly against the Northwoods Software License Agreement for production, some developers use technical workarounds for internal testing: How to remove the upper left corner watermark #130 - GitHub
: License keys are specific to major and minor versions (e.g., a 2.0 key may not work for 2.1). If you upgrade your library version, you may need to request a new key from the GoJS activation page Google Groups Specific Deployment Cases Gojs Remove Watermark - Google Groups 4 Aug 2024 — gojs remove watermark
Run your app. The corner watermark is gone. You can now export diagrams as PNG/PDF without the stamp. gojs remove watermark