How common is "gesundheit"?

Only if people know what gesundheit means. Clearly, not everyone does.

The ‘joke’ depends on the word supposedly sounding like a sneeze, so it doesn’t matter whether the response is bless you, gesundheit or any other well-known term used after someone sneezes. I don’t think any of the example on that page llustrated characters really thinking that the other character had sneezed. It’s a crap joke, but not quite as crap as that article seems to think.

And for all of you Patch Adams fans: Patch Adams - Wikipedia