Fact is, the Gurtner is and always was a cheap carburetor. This was fine when they were in production, as it was inexpensive to get parts, or just replace it outright. These days, the problems are more apparent. I don't think they were ever meant to last 30+ years.
You're right - there is no sealing gasket on the jet - it just relies on a proper fit between the brass jet and potmetal carb body. This is fine once or twice, but with repeated disassembly, it can fail. Even the British would have had the sense to use a fiber washer in this area. What you did should work just fine; I would have tried a little teflon tape in the sealing area, but whatever works here is fine.