In reply to Paul Hy:
Interesting question! To turn this the other way around, I can vouch for seeing Grossglockner from Germany, Czech Republic, Slovenia, Italy, Austria, and Switzerland.
So which are the other three?
I have my doubts about whether Hungary and Slovakia would work, both are rather low lying countries (or, more precisely, the Slovak mountains are on the wrong end of the country). Can you see GG from Vienna?
France? The furthest mountains I could just about make out from Grossglockner was the Valais Alps, about 100 km to the East of MB, so it might just about be possible.
Liechtenstein should definitely work, it has a few 2200m peaks on the Austrian border, depends on whether the view towards East is free from one of them. The Ötztal Alps at least seem to be far enough south to not obstruct the view, and GG has plenty prominence over the Tuxer and Zillertal summits.
GG should also be visible from Croatia (there are reasonably high mountains near Zagreb that should offer a good view NW).
Maybe it is therefore also visible from NW-most Bosnia?
In my experience, though, the mountain in the Eastern Alps that is easiest to spot from far away is Grossvenediger, even if it is 120m lower than Grossglockner.
CB
edit: The viewfinder panorama for GG shows Ucka in Croatia, so that seems to be conformed:
http://viewfinderpanoramas.org/panoramas/EUR/GROSSGLOCKNER-S.gif
Post edited at 18:39