One way to check the dependencies of tc-4.x extensions in tc-5.x is to download and mount the extension squashfs in tc-5.x: $ mkdir /tmp/test /tmp/image
$ cd /tmp/test
$ wget -c http://repo.tinycorelinux.net/4.x/x86/tcz/gmpc.tcz
$ sudo mount gmpc.tcz /tmp/image
$ ldd /tmp/image/usr/local/bin/gmpc | grep png
libpng12.so.0 => not found
..so to answer your question 1, yes gmpc would need to be recompiled in tc-5.x against the updated libpng and the updated gnutls.
To answer question 2, gob2 does not depend on libpng, but libunique does depend on libpng and so would need to be recompiled for tc-5.x.
It's effectively already answered, but for question 3, yes an update to the gmpc extensions is required - the gmpc source should find and link against libpng16, but the older the source the more likely it is that there may be problems.