[...]
Matthias also responded requesting:
I see that both the runtime library and the -dev packages have
different package names. But to be able to still use gmp3 for
existing GCC versions, please change the source name too, such
that gmp3 is still available in unstable after the upload of gmp5.
Shall I go ahead and upload the source gmp5? Then both gmp versions
will co-exist in the repository and packages can choose to move to
gmp5 at their leisure.
After some further investigation, it looks like this isn't feasible.
Neither gmp 4.3.2 nor 5.0.1 version their symbols and with both versions
in the archive simultaneously and co-installable, there's a reasonable
risk of a process ending up with both libraries loaded in to its address
space, which is generally not a good idea.